The terminology and definitions as they relate to gender variations are fluid and changing. Here you will find some, but not all currently used terms.
Cross dressing is simply wearing clothes that are usually associated with a person of the opposite sex or gender. It does not determine ones sexual orientation. This definition starts to become difficult when one scratches below the surface, but is generally thought to be well understood.
This term was coined by Magnus Hirschfeld in 1918 in his book "The Transvestites: An Investigation of the Erotic Desire to Cross Dress." Ever since, it has been associated with fetishism and erotic motivation. This was reinforced by later studies which set out to categorize people according to the "degree of erotic enjoyment they experienced".
There is considerable argument about the use of the word transvestite. Its origins as a definition of treatment for severly disturbed people give the label an unpleasant ring for many people who have this label applied to them. Cross-dresser is a less pejorative term that is acceptable to most people who do not specifically identify as a transsexual.
The word transsexual was first used by Dr Harry Benjamin to describe a particular type of cross dresser who wanted to live life in an "opposite gender role" in the early 1950's. This type of person was distinguished from the class of transvestite by having no apparent erotic stimulation from cross dressing and this was considered a more "worthy" cause than the supposedly sexually "deviant" feelings of the transvestite.
Transsexuals are often viewed as a separate class of people by comparison with others with gender variance but may also be seen as another form of expression of an underlying discomfort with the sex and gender assignment and roles they received in childhood. The one clearly distinguishable feature that identifies transsexuals is their desire to modify their bodies with hormones and sometimes surgical procedures to make their bodies more like they feel they should be.
The term "transgender" has become an umbrella term encompassing a wide variety of those who consider themselves to be one of the "gender variant". The term transgenderist was first coined by Virginia Prince, one of the co-founders of TriEss to differentiate somebody who wished to live their lives as the opposite gender, but has no desire to physically change their bodies. It recognizes that "sex is between your legs, gender is between your ears".
A person who is born with indeterminate or missing male or female sexual organs, either internal or external, or a chromosomal combination other than XX or XY or 47 instead of 46. It is difficult or impossible to assign a "binary" "It's a boy!" or "It's a girl". Sometimes a doctor may advise the parents to have a surgical correction done so the child appears either male or female. When the child matures, their brain gender or identity may be incongruent with the surgical choice. Being mis-gendered at birth can often cause major psychological distress and at its worst could lead to suicide.