Transformers

Transformers is a line of robot action figures that transform from humanoid shapes to any number of not-so-humanoid shapes, from semitrailers to jets to cassette tapes to dinosaurs. Owned by Hasbro, Inc, the toyline was first introduced in 1984. Since then, it has gone on to spawn a number of different cartoons, comics, and movies. Originally aimed at young boys, the toyline is now also enjoyed by grown men (usually the grown-up young boys it was originally aimed at), grown women, and adolescent females, the latter most likely thanks to the 2007 movie.

In Canon
Because of the many different Transformers continua, Transformers canon is hard to pin down- basically, as long as Hasbro approved it, it's canon for some  continuum, and if two continua that are supposedly related contradict one another, both should be considered equally canon unless Hasbro says otherwise. Each continuum is split into several related but slightly different continuties, leading them to be called "continuity families" by the fandom. Characters, settings, and events are often subject to change. However, the basic premise of Transformers canon remains the same in all but a small handful of them.

The Basic Premise
The evil Decepticons and the heroic Autobots are pitched against each other in an all-out war between good and evil. Said war usually begins on their homeplanet of Cybertron, and more often than not makes its way to a certain blue and green globe known as Earth by the natives. There, humans help the giant robots wage their battles, usually siding with the Autobots, as the Decepticons tend to squish them without second thoughts.

Notable Continua
Some Transformers continua are more popular than others, and thus are more used in fanfiction. A full list of continua and their relations to one another exists at the TFWiki, linked at the bottom of the page.

Generation 1 continuity family
The very first Transformers continuum ever. Made up of the toyline (duh), comic books, and, most notably, a cartoon series that lasted from 1984 to 1987. Other members of this continuity family include the less successful Generation 2 series that lasted from 1993 to 1995, as well as the Beast Wars series that lasted from 1996 to 2000.

The Unicron Trilogy
Comprised of three series, Transformers Armada, Transformers Energon, and Transformers Cybertron, three Transformers anime series, known in Japan as Transformers: Micron Legend, Transformers: Super Link, and Transformers Galaxy Force respectively. It is named the Unicron Trilogy after the unifying theme of the threat of Unicron, a planet-eating dark god. Unusual in that while Energon follows Armada on the timeline as a sequel series, whether Cybertron follows after Energon or its place on the timeline is "off to the side" (so to speak) is debated. Notable for things such as Armada's incarnation of Starscream being possessed in an infinite number of badfics, Energon's notorious lack of quality, and the Cybertron dub being bashed by people who saw the Japanese version first in spite of the fact that on examination, it is of higher quality than Armada and Energon, and overall works better as a series than the original Japanese (so says TFwiki).

2007 Movie
Set in modern times, this live-action movie is probably responsible for most of the teenage girl Transformers fans. A sequel came out summer 2009. The third is set for summer 2011 release.

Transformers Animated
A more recent version of the series, formerly viewable on Cartoon Network. Unlike previous cartoons, it's actually enjoyable for adults to watch, as well as young children. It lasted from 2007 until 2009.

Aligned continuity family
The most recent version of the series, designed by Hasbro to be the foundation of the franchise for the next decade. Starting with the War For Cybertron video game and continuing with the cartoon series Transformers Prime, it combines aspects from the other continua together in an effort to to form a more unified continuity possessing the best qualities of its predecessors.

Characters
Because the purpose of the Transformers toyline is, well, to sell toys, there are many, many, many, many more characters than those listed here. These are the ones most used in fanfiction.

Also, because Hasbro's aim, like most corporations, is to make as much money with as little effort possible, many characters have several different versions of themselves--colors, personalities, alternate forms, and abilities vary from continuum to continuum.

Autobots

 * Optimus Prime
 * Bumblebee
 * Prowl
 * Jazz
 * Ironhide
 * Ratchet
 * Sideswipe
 * Sunstreaker
 * Bulkhead
 * Mirage
 * Hound
 * Bluestreak
 * Elita One
 * Chromia
 * Arcee

Decepticons

 * Megatron
 * Starscream (the undisputed little black dress of the fandom)
 * Soundwave
 * Skywarp
 * Brawl
 * Bonecrusher
 * Barricade
 * Frenzy
 * Blackarachnia
 * Lugnut
 * Thundercracker

Humans

 * Sparkplug Witwicky
 * Spike Witwicky
 * Sari Sumdac
 * Isaac Sumdac
 * Sam Witwicky
 * Mikaela Banes
 * Reginald Simmons
 * Tom Banachek
 * William Lennox
 * Sarah Lennox
 * Annabelle Lennox
 * Robert Epps
 * Maggie Madsen
 * Glenn Whitman
 * John Keller
 * Seymour Simmons
 * Alexis Thi Dang
 * Jack Darby
 * Miko Nakadai
 * Rafael "Raf" Esquivel
 * William Fowler
 * June Darby

In Fanfiction
Again due to the number of different continuities, Transformers fanfiction is quite varied. Oftentimes, writers will introduce characters who were prevalent in one continuum into another where they are never or rarely seen. Writers may also choose to further blend two or more continua together (which admittedly has happened in canon on occasion, as the many continuity families are considered to be different realities that all exist within a single multiverse...except for the Aligned continuity family; it's really complicated). It can get very confusing for those not familiar with the toyline.

Notable problems in Transformers fanfiction include bad slash (They're giant robots. They don't interact the way we do.), interspecies romance (Repeat: giant robots. Falling in love with a human is unlikely. It must be pointed out, in the interests of fairness, that this did actually happen a couple of times in the TV series, but even so....), and the occasional Mary Sue (available in both human and robot forms).