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In countries where voting is compulsory the first option (nullifying your vote) will probably let you abide by the law, while the second might not. E.g.

Former Australian opposition leader Mark Latham urged Australians to lodge blank votes for the 2010 election. He stated the government should not force citizens to vote or threaten them with a fine. At the 2013 federal election, considering the threat of a non-voting fine of up to $20, there was a turnout of 92%, of whom 6% lodged either informal or blank ballot papers.

If the voting is electronic you probably won't be able to nullify your vote though. Also in some countries with "compulsory" voting you just have to show up, you don't have to actually vote... which again makes the choices equivalent, except that you have to show up to just not vote.

Also even in countries with non-compulsory voting some just write/draw stuff on the ballot as a protest, perhaps also hoping it will get picked up by the press [NSFW!].

I will provide a complementary answer for the more general interpretation of the question in your title:

What is the difference between nullifying your vote and not going to vote at all?

If the voting refers to a referendum it might actually make a difference. E.g. Today Romania also held a referendum (along with the EU Parliamentary elections). By law in order for it to be validated it must have:

• a voter turnout >= 30% registered voters


• 
  valid votes >= 25% registered voters

Let's assume about 71% of the voters are acting consistently:

• if they all come and nullify their votes, the referendum is valid


• if they do not come to vote at all, the referendum is not validated

When you do not vote, you are basically saying "everything is fine, no matter who wins."

When you spoil your vote, you are either saying "none of the above, and things are not fine" or "oops, didn't read the instructions." Unfortunately the last two can be hard to tell apart from the published election percentages, but if there is a significant movement before the election that tells people to spoil, the percentages will reflect this.

In some jurisdictions the counting is public, so a group of interested citizens could watch and count how many spoiled votes appear deliberate.

In most systems there is no substantial difference.

In the UK system, blank ballots (along with other invalid ballots, such as people who vote twice, or sign the ballot paper) are counted as "spoiled" and when the result is announced the number of votes for each candidate is read out, along with the number of spoiled ballots. The percentage isn't particularly important, it is the candidate with the greatest number of votes who wins.

Even if there are more spoiled ballots than valid ones, it doesn't affect the result. The spoiled ballots are counted, but not counted for or against any candidate.

Political analysists may later calculate the percentage of valid votes, or the percentage of all votes, or the percent of the registered electorate, usually to form a particular narrative.

Scenario 1 (100 registered voters)

A 40
B 35
C 15
Spoiled 10

Candidate A has 40% of the electorate and 40% of all votes, but 44% of valid votes

Scenario 2 (100 registered voters)

A 40
B 35
C 15
Spoiled 0

Candidate A has 40% of the electorate but 44% of all votes, and 44% of valid votes.

The outcome is that candidate A is elected. The end result is exactly the same, and so there is no substantial difference.

Maybe those people were talking about blank vote, not null vote.

There are two main types of protest voting: null voting and blank voting.

Null voting is when your ballot has drawings on it, is defaced in some way, has more candidates selected than it should, is not an actual ballot (e.g. you put a picture of Donald Duck inside the envelope), etc.

Blank voting is when your ballot has no marks, or a "None Of The Above" mark where available.

In most electoral systems, a null vote is not taken into account, so it is the same as not voting. The difference lies in null vote vs. blank vote, and it depends on the electoral system.

There are basically three types of electoral systems:

• 
  Plurality systems, where the winner is the one who gets more votes (not necessarily a majority).


• 
  Majority systems, where the winner is the one who gets a majority of the votes.


• 
  Proportional systems, where the amount of representation of each candidate is calculated based on the amount of votes they get.

For plurality systems, it doesn't make a difference: if candidate A got 10 votes, candidate B got 9 and candidate C got 8, then candidate A wins because that was the candidate with more votes. Votes not going towards a candidate are ignored.

This system is the one used in the US, the UK and countries influenced by them.

For majority systems, it depends on whether blank votes are counted as valid or not. If they are considered valid votes, blank votes can make it harder for any candidate to reach a majority: if the voting goes A 10 votes, B 5 votes, C 2 votes, blank 2 votes, A gets a majority and wins; but if there were 5 blank votes instead of just 2, then A didn't get a majority of valid votes and probably a second round between A and B would be needed. This doesn't necessarily benefit any candidate, since C's votes from the first round could go to either of them or maybe turn to blank votes.

This system is used in e.g. France, where blank votes are not valid votes and thus ignored (candidate A would win in both scenarios).

For proportional systems, null votes are ignored, but blank votes are usually not: they are valid votes and so are taken into account when calculating representation. Blank votes in proportional systems tend to work against candidates with less votes, by making it harder to reach the minimum % of total votes needed for a seat, or by altering the coefficients used when assigning seats, resulting in candidates with a higher number of votes being assigned more seats than if no blank votes had been cast.

This is the system used in most European countries.

TL;DR: Null votes usually don't make a difference, but blank votes can do a big one (in most of Europe at least). Maybe that't what you heard some people talk about.

A person who did not vote for A or B may be:

• Opposed to both A and B


• Not aware there is a vote


• Too busy or lazy to go vote

Ordinarily, it is not easy to decide on the proportions of these. But nullified voters are clearly aware of the vote and care enough to show up. Therefore they can be an indication of whether both choices in a vote were poor. For instance, if you have 10 million people, of whom 2 million vote for Alice, and 1 million vote Beatrice, you might conclude that the population favors Alice. However, if you also get 6 million null votes, it is clear that Alice would not make people much happier than Beatrice, so maybe it's worth re-doing the vote and include Charles this time.

It also makes fraud a bit more difficult, of course. Once somebody casts a vote, it's pretty easy to detect a false vote in their name - there would be duplicate votes. The easy way to do vote fraud is to find people that won't vote (whether because they don't care, or you paid them not to, or you destroyed their ballot box, or they're dead) and then vote in their name. If somebody doesn't want to support either option, but also wants to make life harder for frauds, then a null vote makes sense.

• If the vote is not on parties but laws (or changing the constitution, or a referendum), there is usually a minimum required turnout for the voting to be taken into consideration. An invalid vote can increase turnout (depending on jurisdiction), while not voting does not.


• Regions or neighborhoods with distinct socioeconomic or ethnic makeup might find that if they have lower turnout they will be ignored by the central government (less investments, less roads being maintained, etc.), as they "matter less". Therefore, someone who is dissatisfied by all of the parties and doesn't prefer any candidate over others, might still go to vote and submit an invalid vote, just to increase turnout in the region/neighborhood.


• If there is suspicion of election fraud, people who don't favor any candidate might be incentivized to show up and submit an invalid vote, in order to make it more difficult for others to submit a fake vote in their name.

Technically they are the same: Invalid votes are the same as no votes at all.

There is still a subtle difference: Both, a valid and an invalid vote, produce the same workload for the poll clerks. They have to open the letter and look what's written on it. If they see that it is a vote that is intentionally made invalid, it just increases frustration for spending time on some nonsense. In this case it would be more fair to not vote at all.

I have spoiled my ballot paper (by writing "None of the above") just once. I did so because I wanted to actively make the point that I believed the election should not be taking place at all.

(It was for the position of elected Police and Crime Commissioner in the UK. I felt the old system where the police was responsible to the local authority was better.)

Most people are getting hung up on laws. But elections are not about laws.

The purpose of an election is to confer a form of legitimacy on the winner of the election -- democratic legitimacy.

The laws and procedures around the election are setup to make the election seem legitimate. In some cases and systems, this also makes the election an actual reflection of people's votes.

Now, not voting is relatively common in wealthy western democracies. It is usually interpreted as apathy -- not caring who wins. A turnout of 40% or 60% or 70% of those eligible to vote may make the news, but as a minor point. Only drastic swings away from a previous vote total are really news worthy.

A massive campaign to not-vote, protesting the election itself as being illegitimate, can undermine the democratic legitimacy of the result of the election. This is done when a portion of society feels that the rules of the election have tipped the scales in favour of one result or another to an unfair degree.

Spoiling your vote sends a different message. Many systems record votes as being spoiled, and in some cases record refused ballots separately.

Spoiled ballots are usually interpreted as the voter making a mistake. In some cases, mass spoiled ballots are used as a sign that the vote counting wasn't legitimate, and triggers a recount.

Mass refusals of a ballot haven't happened as far as I know. But, in a system where it is or can be tracked separately, it would be a really clear indication that the ballot wasn't considered legitimate by the people getting ballots.

An example of where that might be used is when the people writing the ballots have eliminated an option that would win. Not showing up would be one option, but showing up and declaring the ballot illegitimate by declining it en-mass might make the result of the election seem extremely illegitimate.

The down side is that the people setting a ballot are usually those who count the ballots, and often have control over the media messaging around the vote. Getting the message out "show up, decline your ballot" and having that accurately counted to the point where the election is clearly illegitimate would be quite a challenge.

TL;DR - voting is a form of power and expression. Not voting and spoiling and declining and voting are all different expressions, and can be interpreted differently. Under most voting systems, only those who vote are counted for the "winner", but the purpose of winning an election is to claim democratic legitimacy.

A country with 30% turnout can have the winners of the elections claim democratic legitimacy.

A country with 60% turnout, where half of the ballots are refused or marked "none of the above", is going to have a democratic legitimacy deficit, even if it picks the same winners.

EDIT: Short answer: it depends on the regulations and law in your country.

In my country, it does not make any difference at all, regarding the ballot itself. Blanks, nulls and abstentions are thrown in the same "invalid" basket and do not elect (or avoid election) of any party or candidate. But, also in my country, if you stay at home you are breaking the law. If you go and nullify your vote, you are a respectable law-abiding citizen.

--

In Brazil you have to vote - not voting is against the law. And the voting is almost 100% electronic, with only some remote areas in the Amazon jungle having to revert to the old paper votes due to malfunction of the machinery involved.

But even so, you have the option of nullifying or casting "blank" votes. Every candidate has a number, you vote on them entering that number in that specific question. The presidential, governor and mayor candidates have two-digit numbers, the senators three-digit, congress representatives four-digit, state-representatives five-digit and city council candidates have six-digit numbers. You can nullify a vote if you enter a number that does not belong to any candidates (and they save the "all nines" number for that). AND, if you want to cast a blank vote instead of nullifying it, there is a special "blank" button in the electronic ballot that you press. They tabulate blanks and nulls in separate collumns for statistical purposes, but under the law they represent the same thing.

Under Brazilian law, only the valid votes are considered for electing someone, nulls and blanks are invalid and have absolutely zero influence on election results. Even if 99% of voters stay at home or blank/nullify their votes, that 1% that went to vote and chose a valid candidate decides the election. But abstentions (which are illegal), blanks and nulls are counted and registered for statistical purposes, and appear on the authorities' reports about the election.

This link brings a Picture of the electronic ballot used in Brazil. It's very simple and quite clever, despite the (unfair) concerns about the data Security of the whole system.
http://www.omorungaba.com.br/upload/fotos/not_1502108394.jpg