Good question, complicated answer. Now pay attention because this gets involved. All transvestites and sissies are crossdressers by definition because they dress up in clothes of the opposite sex, but not all crossdressers are transvestites or sissies.
A crossdresser may be a transvestite or sissy, or even both depending on the day, weather conditions and heels available.
Remember though, that these are just labels. If a boy wants to dress up as a girl he can call himself anything he wants. He can simply dress up and prance around the house for a while, or dress up and go out to a bar to look for a boyfriend.
There is one little known way to tell the difference with 85% accuracy, if for some reason you feel the need to know. A crossdresser tends to take better photos.
This has yet to be scientifically proven, although not for lack of volunteers. The scientists tend to be crossdressers as well as transvestites with sissy tendencies, and most of the photos end up all wet and sticky.
For more psychological theories of crossdressing and fetish you can listen to The Psychology of Crossdressing here.