TG (short for Transgender, but implicitly, Transformed Gender) refers to a type of transformation where the target is a person of one gender transforming into a member of the other gender; typically through magical changes. It also refers to pictures, usually fanart, depicting a character of one sex/gender as another.
Many TG works are non-sexual in nature and done for creativity or simple humorous purposes. Since this is especially true with characters displaying heavily stereotypical manners for their gender, TG art is predominantly of males. In many TG works, the TG'd victim becomes so satisfied with their new forms that they decline to change back, even if a cure is readily available.
TG Art and Literature can also involve the 'victim' being involved in sexual situations as a member of the opposite sex. A typical plot can involve a person becoming the opposite sex, engaging in various forms of sexual activity. and eventually becoming so satisfied with their new forms that they decline to change back, even if a cure is readily available.
Fan Art Edit
Many TF artists choose to draw famous or popular characters who will still be recognizable to the audience, modifying their traditional costume only as much as needed. Some online communities, such as 4chan, refer to it as "Rule 63" or "Rule 75" (especially when the image contains nudity).
Some artists consider it a cheat to draw TG art of certain characters who have canonical androgynous overtones to them. Guilty Gear's Bridget is very rarely drawn as explicitly female, while characters like Final Fight 's newhalfs Poison and Roxy tend to be more popular in futanari art.
For anime fans, Ranma ½ popularized the idea of a magically induced transformation with few or no other associated physical issues, and were common avatars in fandom.
Demitri Maximoff of Capcom's Darkstalkers series popularized TG art of video game characters (especially from fighting games) which in this case is often called MB. This refers to an in-game special attack of his called Midnight Bliss, wherein the opponent is turned either into a woman (if male) or into a more stereotypical woman (if female). Thus since "official" TG versions of several characters exist, fan art typically speculates and extends this idea to other characters.
Final Fantasy VII may also have contributed to TG in video game fan art, as Cloud Strife (who once had to wear a dress as a disguise) has been variously depicted not only as a crossdresser but also as an actual girl.
Male Naruto characters are often portrayed as female in fan art, referring to the "Oiroke no Jutsu", a technique created by the title character that allows him to transform himself into a voluptuous, naked woman as an illusionary tactic.