What is the difference between "gender" and "sex"?

What is the difference between gender and sex? Wiktionary says that gender is The mental analog of sex but that's too high English for me.

Basically, I'm developing a web-application that stores people's profile along with a field for gender/sex. Which name would be more appropriate?

__

[UPDATE]

After getting these good answers, I can figure out that,

1. Sex - Biological characteristics
2. Gender - Social characteristics

And, well, for my web-application, I went with the term sex, as it sounds more scientific and less political. P.S. Values for sex are clearly defined and can be easily validated (only two values AFAIK* + an optional blank value).

_{* plus some anomalies.}

Solution 1:

According to APA style, "gender" refers to culture and should be used when referring to men and women as social groups, while "sex" refers to biology and should be used when biological distinctions are emphasized.

Solution 2:

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. :-)

Solution 3:

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.

Solution 4:

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.