A transvestite (also seen shortened to tv or TV) is someone who regularly or occasionally wears the clothing of another gender, normally because the dresser wants to pass as a member of the opposite sex.
Transvestism can be seen as a deeper attachment to the element of cross-dressing which is just wanting to wear articles of clothing associated with the opposite sex, but not emulate the opposite sex. See Bobette and Eddie Izzard. ;-)
Some use the terms interchangeably, but many TVs feel that they want more than just to cross dress.
A transvestite in the feminisation realm is often called a sissy or a sissy maid.
Transvestism should not be confused with homosexuality; most TVs are not homosexual and vice versa.