Transgender: A person born feeling they are of the wrong gender.
Transsexual: A transgender who decides to start transition.
Pre OP Transsexual: A transsexual who has yet to do sexual reassignment surgery.
Post OP Transsexual: A transsexual who has had sexual reassignment surgery.
Non OP Transsexual: A transsexual who feels they are of the opposite sex, but has no interest in sexual reassignment surgery.
MTF (Male to Female) Transsexual: A Male who feels they are actually a female and is in transition.
FTM (Female to Male) Transsexual: A Female who feels they are actually a male and is in transition.
Transition: The name for the process in which a Transsexual changes from one gender to the other. Involves dressing full time as the gender, Hormone Replacement Therapy, and for some Sexual Reassignment Surgery.
HRT (Hormone Replacement Therapy): A process of blocking the bodys natural hormones and supplementing them with the other genders hormones.
SRS (Sexual Reassignment Surgery): The final stage of transition except for Non OPs. Involves turning the current genitals into the other genders genitals.
Gender Identity: The gender the person most identify with.

Transvestite: A man or woman who still feels they are a man or woman but likes to cross dress in the other genders clothing and gender bend.

Important to note that MTFs can be lesbians, and FTMs can be gay, gender identity and sexual preference are not linked.

Shemale and Tranny are derogatory terms for transsexuals and transvestites respectively. They are terms normally only used in the porn industry, though there are some in either group who dont mind being called it.