Transvestites are cross-dressers who cross-dress because they get a sexual kick from it, and most of them are straight and are as horrified by the idea of having sex with another man as is any 'normal' man.
Transsexuals are totally different however and they believe that nature made a mistake and that they are women living in men's bodies, and they will go to very extreme lengths to get out of them.
How far will they go?
Well to become a woman they must go through a long and difficult transition including hormone treatment, painful surgery which involves having their penises removed and a vagina constructed, followed by lots of therapy to help them adjust to their new bodies.
Most transsexuals say that they are happy with the final results but would never have done it, if they'd known how difficult and painful it would be.