What is the difference between sex and gender?

A lot of people use these two words interchangeably, or use gender instead of sex because it sounds nicer or avoids confusion.

But there is a useful distinction between sex and gender.

Sex is biological.  It's about sex chromosomes, primary and secondary sexual characteristics, and who gives birth. 

Gender is social.  It's about what biological sex means to us, about our expectations and behaviour.

Of course, the line between the two is the subject of much debate.

But if you look make the distinction, you can see that you have very little choice about your sex.  Surgery or hormone therapy can alter some sexual characteristics but cannot change your sex chromosomes or recreate the fertile sex organs of a different sex.

But gender is another case altogether.  How you act and dress and relate to people may be influenced by biological factors but is also the result of what you want to do.