Pre-op TS - A transsexual who has not yet undergone the surgery required to complete a physical change of sex, although may be in a transition period in which he or she is living full-time as a member of the "opposite sex" and recieving hormone therapy to partially facilitate the physical sex change. Normally implies the individual is planning to have SRS.
Post-op TS - A transsexual who has completed SRS (Post operative), the physical change of sex surgical procedure. No longer considered by some as transsexual, however since gender surgery is never quite the same as having the appropriate gender at birth most individuals still accept that they are transsexual.
Non-op TS - A transsexual who is not planning or is not able to have sexual re-assignment surgery, but may still live and work and dress in role of opposite gender
Transsexual (transexual) or TS - To be Transexual means that you fundamentally view yourself as being of the opposite sex to your physical body. You could have a male body with a female mind, this is called Male to Female Transexual (often abbreviated MtF or MtoF). There is also the reverse, a person born with a female body and a male mind. This is Female to Male or FtoM. In fact this condition affects not only Humans but animals too. Although it's exact causes are yet to be conclusively understood, current medical knowledge classes Transexualism as a birth defect. A part of the individuals brain which deals with gender is formed as that of the opposite sex to the physical body. Fortunately this condition is relatively rare and whilst transexualism certainly isn't the norm, it is a natural phenomena and definitely isn't invented or "chosen".
You may also hear the terms pre-op and post-op Transexual. This is because some transexuals find living with their condition so difficult they take hormones and undergo surgery (SRS or Sex Reassignment Surgery or GRS Gender Reassignment Surgery) to make their body match their mind. This process is called transitioning. Whilst current medical science only offers a sex change as a cure to transexualism, the degree to which a person has mastery of their own mental states often has a bearing on whether a sex change operation is essential. Some will get depressed or view their life as a lie if they are forced to live in the gender role of their physical body and in extreme cases sex change can be the difference between life and death.
However it would be wrong to say that the goal of all transexuals is to transition to one gender. There are many more who leave their bodies unaltered and view their unique blend of female outlook with masculine strength and presence as an advantage. Certainly statistically speaking, many transexuals benefit from this combination of female understanding with male competency, as a higher than average number of us are in the higher income bracket with technical or professional skills. Oh come on, you know when God takes with one hand he gives with other!
A shemale is someone who takes hormones and may undergo feminising surgery but chooses to keep their male genitalia. Some do this because it allows them to earn money in the sex or escort industry since once they have the chop, they loose their uniqueness. This allows them to save money for their treatment as transitioning can be very expensive with surgery alone costing thousands, not to mention the cost of hormones which transexuals must take for the rest of their lives.
Similar names include Chick with a Dick and Ladyboy. Whilst Shemale is a word originating in the porn industry, Ladyboy is a translation of a Thai word as Thai people have believed in the existence of a third gender for thousands of years. Not all shemales transition like transexuals do as some prefer being both sexes or have commercial interests that require them to have a functioning male genitalia.
Transvestite, tranny or TV - A transvestite is a person who chooses to outwardly express their inner feelings through their appearance. They could be male or female yet have the desire to show their feminine or masculine nature through dress. Think of it this way. You have a line numbered from 1 to 100. 1 is 100% female and 100 is 100% male. The majority of people would be a comfortable mixture of both genders in the way they think - say 90% male, 10% female. However, some people have a strong feminine or masculine side, maybe 60-40. Having this stronger leaning within they may feel unable to express themselves fully within the constraints of their day-to-day gender and so find release in spending time within the opposite gender role.
By far the majority of Transvestites are heterosexual, marry and live otherwise "normal" lives save the odd girls night out at a TV club or group or they dress at home. The majority of Transvestites do not take their need for expression into their sex lives and topics on discussion at TV groups are normally very clean, along the lines of a woman's magazine - make up, dressing nicely, where to shop for bargains etc.
Cross Dresser or CD
Cross Dresser or CD - Whilst most Transgendered groups could be considered as a form of cross dressing, within the TG community Cross Dressing is often seen as something more specific; it's when dressing is done for sexual pleasure. It often centers on wearing more intimate apparel such as lingerie or underwear, as well as fetish wear like PVC or leather. There can be a huge gulf in outlook between a Transvestite and a Cross Dresser since the Cross Dresser is motivated by sex, whilst the transvestite is motivated by the need for expression. Therefore Cross Dressers may not feel the need to look convincing or act in anyway like a female would.
For some cross dressing can be part of a complex about their sexual orientation - some people cannot accept themselves to be bisexual or gay and so cross dress because the thin veil of the woman's role allows them to act out fantasies that their own mental conditioning is at odds with. It's basically like saying to yourself it's OK for me to be having sex with a man because at the moment I'm a woman.
There are also cultural differences in this term. In North America, the word crossdresser can essentially be the same as transvestite; it needn't necessarily connotate for sexual purposes. However in Europe there are clear examples of this connection. For example if a celebrity or politician was found in some sex scandal where they were dressed it would be described as a "crossdressing shocker" rather than a "transvestite shocker". This is why I prefer to split the terms transvestite and crossdresser to refer to two different motivations.