The canon dates to 1984 (G1). I decline to try to explain 35 years of comics/cartoon/movie/etc toy-selling canon in any detail.
Major canons (per AO3):
Transformers Generation One: Roughly the 1984 - 1992 canon.
The Transformers (IDW Generation One): The IDW Comics reboot of G1 canon, running from 2005 - 2018. The Lost Light run is included in this canon. IDW's 2019 comics are in a separate reboot, which fans may or may not mash into their fic.
Transformers: Prime: the 2010 - 2013 Hub Netwook cartoon series and associated media. The first season is on Netflix. Fringe fans take note, Roberto Orci and Alex Kurtzman show up in the credits as executive producers. This may or may not have had any bearing on the cartoon series' complete lack of shame about the TV Tropes checklist it works through. If you're going to do a quick sample skip to the "One Shall Fall / One Shall Rise" arc. Those fics where Optimus Prime gets amnesia and thinks Megatron is his BFF who saved him from the wicked autobots? Canon.
Transformers Animated (2007): The 2007 - 2009 Cartoon Netwook series and associated media. Notable for plucky little young Autobots menaced by big bad Decepticons from half-forgotten history, which plays to the fannish id not at all. Cough.
Major players:
Optimus Prime, leader of the Autobots (good guys) (usually) (mostly) (except for Shattered Glass, which is exactly the mirrorverse trope you think it is). Optimus is frequently found in the company of:
Ratchet, autobot medic and sometimes scientist. Flavors vary from early G1 party-loving Ratchet to a cynical, war-weary Dr. McCoy type.
Prowl is sometime ex law enforcement, rarely a ninja, occasionally the Autobot's strategic go-to guy, and always the calculating, logic-driven party-killer of Team Optimus. Often contrasted with...
Jazz, life of any party, an on-his-pedes thinker who absorbs Earth culture, slang, and music. This makes him no less deadly on the battlefield; in G1 continuity he's the go-to tactical guy countering Prowl's patient, logical calculations.
Tiny, good-natured Bumblebee does not let being either small or nice get in the way of kicking Decepticon aft. Bee often quickly bonds with humans who encounter the Autobots. In several canons he's lost the ability to speak "normally" and works around this through incomprehensible-to-humans Cyberton languages and/or pop culture.
Megatron is the leader of the evil Decepticons. Depending on the flavor of canon, Megatron is a tyrant, an ex-gladiator turned warlord, or an ex-agitator for social reform who razed his enemies' cities to the ground and founded the Decepticon Empire. Sometimes a canon poet.
Starscream usually shows up high in the Decepticon hierarchy: Air Commander, second in command, etc. The only things more impressive than his flying skills are his ambition and lust for power, which lead to periodic schemes to overthrow Megatron and declare himself leader of the Decepticons. Sometimes also a pre-war scientist.
Where Starscream blusters and schemes, Soundwave is tirelessly loyal to Megatron and the Decepticon cause. May or may not be able to read minds; in most universes he's associated with the cassettes, variously including Ravage, Laserbeak, Buzzsaw, twins Rumble and Frenzy, and many others.
Galvatron and Cyclonus don't squeak into AO3's Transformers - All Media tag, but highly relevant to the Astolat binge I know certain people are considering. So! Galvatron is the Decepticon leader who has more firepower than Megatron (because he was built out of Megatron, only better, in some universes) and even less sanity (because the Autobots almost managed to kill him, but merely succeeded in brain damage, in some universes). Cyclonus is the Decepticon second in command who balances out Galvatron's madness, or is crazier than Megatron, or is a sort-of-non-Deception ex-sort-of-zombie on the Lost Light road trip. *shrug* We could get into the backstory, of course - OH LOOK CYCLONUS AND TAILGATE ARE FLIRTING AGAIN.
Ahem.
There are female-identified Transformers as well, but the canon and AO3's biases both tend to leave them kind of on the side.
Major pairings:
Best Enemies: Optimus/Megatron
Opposites Attract: Jazz/Prowl
Sleeping with the Boss: Megatron/Starscream
Canon gay: IDW!Rachet/Drift
( Sample fics )