Drag queens -- men who perform in women's clothes. It frequently has no connection to sexual gratification or gender identification. They are almost always gay or bisexual. I was friends with a drag queen years ago who was married and the father of two children.
Drag kings -- women who perform in men's clothes. However, they usually seem to be more "full time." I went to a party recently where there were six or seven present -- the kind who had cultivate little wisps of hair on their chin into goatees.
Transvestites -- men who cross dress because they derive sexual gratification from it. They are often heterosexual.
Transsexuals -- members of either gender who, having a conviction they belong to the other sex, undergo sex reversal typically or live as a member of the opposite sex. I have recently been working with the family of a 5 year old boy who is convinced he is a girl and has identified with them since he was an infant.
Transgendered -- refers to any of the folks who cross gender lines.