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There's a lot of foofarah coming from people in the GLBT communities trying to establish a difference of meaning between these two words that are, in ordinary usage, completely synonymous.

Wiktionary's definition is trying to reinforce that distinction by saying, essentially, that "sex" is what physical parts you have, while "gender" is what sexual category you think you should belong to; thus allowing for males who wish they were females to claim that their sex is male but their gender is female.

For your web application, you can use whichever one you feel is more appropriate. Personally I recommend "sex" because it's only 3 letters long, saving you some keystrokes if you reference the field a lot. :-)

The NOAD defines gender as "the state of being male or female (typically used with reference to social and cultural differences rather than biological ones)."

A further note in the dictionary reports that:

The word gender has been used since the 14th century primarily as a grammatical term, referring to the classes of noun in Latin, Greek, German, and other languages designated as masculine, feminine, or neuter. It has also been used since the 14th century in the sense "the state of being male or female," but this did not become a common standard use until the mid 20th century. Although the words gender and sex both have the sense "the state of being male or female," they are typically used in slightly different ways: sex tends to refer to biological differences, while gender tends to refer to cultural or social ones.

In this context, I prefer to see the word gender as the label with the two choices offered as alternatives, which makes the meaning quite clear.

Although both have an equivalent meaning in this context, I find gender to be more polite as it cannot be confused with intercourse which is an alternative meaning for sex.

i.e. Two people would not have gender with each other, but they might have sex.

Male and female are sexes, while masculine and feminine are genders. Sexes are nouns -- "I am a male, my wife is a female". Genders are adjectives -- "She has a masculine voice".

You can, of course, say "She has a male voice", but it means something subtly different. "She has a masculine voice" means her voice shares some of the characteristics we associate with being a man. "She has a male voice" means that her voice is in fact a man's voice. (Though it would certainly be interpreted metaphorically.)

Sex and gender are two words used interchangeably. Sex refers to the the biological distinction between females and males, which is closely related to reproduction, while gender refers to the social attributes and opportunity associated with being female and male.

Sex refers to the physical sexual organ parts that a human being has. Gender on the other hand refers to the sexuality that a person identifies themselves with.

The choice of term used probably would not matter to a majority of the users, but it is a big deal for certain people. For instance, deviantART used to offer three choices on their user profile field - male, female, and 'unspecified'. They removed the last option in 2010 and it caused widespread outrage among a subset of users, with vocal calls for boycotting deviantART too.

Since listing a range of choices under 'gender' itself can be topic of contention (e.g., what if you 'miss out' a certain gender identity in the list?), one way some web application developers get out of this is by offering an "I do not prefer to say" or "Unspecified" option under a sex field.

Well, if your app is going to be used worldwide, then the choice that you make could potentially have a huge impact. Have you heard of the third gender? Some countries such as Pakistan have officially recognised a third gender amogst their people. The idea may sound bizarre to some western ears, but that's the world for you.

If you are interested in recording biological characteristics, use "sex". If you are interested in social ones, use "gender". Everything else is silly political correctness or plain misinformation.

Isn't "gender" referring to two separate ideas: 1. the biological one (=sex) 2. the technical term in sociology, so you can of course use it synonymously with sex?

There are lots of answers, but I'd like to add mine in here, because I strongly disagree with most of the others. The current usage for the younger generations is precisely the reverse of that of our grandparents. @yc01 sums it up well, by putting it exactly backwards to how it used to be just a little while ago. It used to be (and I would go still with this) that sex is a social term, and way of referring to people, whereas gender is the scientific word, the sort of thing used to describe lab rats or people you're dissecting, or a taxonomy on some government paperwork. I avoid referring to someone's gender; it can be a slightly horrid, impersonal word.