Considering that Transformers have been in almost continuous production as toys, cartoon characters and comic book stars since their creation 25 years ago, it's not surprising that they've undergone numerous relaunches and changes in appearance. If you can't tell your G2 from your Beast Machine, or you're wondering which version of Optimus Prime turned into a gorilla, here's a quick guide.
1984-1992: Generation One
The original, and still the best. This is the incarnation that most people still think of when someone talks about Transformers - the one where giant robots turn into vehicles, dinosaurs, cassette tapes and all manner of other unassuming objects when they're not beating the crap out've each other. It's also the one that brought the world this classic piece of music:
1992-1995: Generation 2
Essentially a reboot of the original toyline with characters and figures based on the originals, but with details and colors changed to make them both safer and, theoretically, more toy-friendly. Theoretically, in the same continuity as the original G1 line, and notable for having advertisements demonstrating some truly basic CGI:
Somewhere, a young Michael Bay saw this and thought "One day... I'll do that. But better."
1996-1998: Beast Wars
A full-out reboot of the franchise (until they started bringing in G1 characters and concepts, but that was retconning the reboot, and things get muddy from that point on), with Autobots and Decepticons replaced by Maximals and Predacons, and Primal and Megatron replacing Optimus Prime and... um... Megatron as the main characters (Honestly, the two Megatrons are different characters). Unlike the majority of the original Transformers, these characters don't transform into vehicles or objects, but animals.
1999-2001: Beast Machines
A continuation of Beast Wars, but this time the Predacons are replaced by the Vehicons, as Megatron replaces his henchbots with... well, new henchbots, but these ones turn into (other) machines, instead of animals.
2001-2001: Robots In Disguise
A curious (and short-lived) mix of G1 and Beast Wars, this line set the Autobots versus the Predacons, who then created the Decepticons out of former Autobots because... Oh, I'm still not sure. All that you really need to know what that the animated series was as if someone had wanted to remake the original for the Pokemon audience, and was every bit as enjoyable as that sounds:
2002-2004: Armada/2004-2005: Energon/2005-2006: Cybertron (AKA: The Unicron Trilogy)
A four year line co-produced by Hasbro and Japanese manufacturer Takara, this revision of the franchise started with Armada hewing closely to G1 concepts, but with the added bonus that certain characters could essentially "power up" by partnering with a power-enhancing smaller robot called a "Mini-Con".
Energon took place 20 years after Armada, with Transformers settling on Earth (including building giant robot cities to live in) and having developed the ability to combine with each other to form even larger giant robots.
Cybertron followed from Energon, but only in America - The Japanese version of the line (called Galaxy Force) was another continuity reboot - and saw the Transformers travel the galaxy looking for "Cyber Planet Keys" that could... unlock a black hole and stop the destruction of Cyberton. Hey, I didn't come up with the idea.
2002- : Transformers Universe
Pretty much a re-release line from Hasbro, Universe has featured toys from every Transformers line repainted and reissued. Lacking a cartoon series of its own, the Universe continuity as such consists of a Collectors Club-only comic that explains that this incarnation sees Transformers from different times and different parallel earths brought together to defend a peaceful, post-Beast Machines-era Cybertron from the invading Unicron.
2007- : Movies
The Transformers go mainstream, thanks to Michael Bay. To all intents and purposes, the first movie is a revised version of the classic G1 continuity, right down to the use of "Whitwicky" as the surname of the human hero... just with added Bayhem. By the second movie, it's beginning to build its own mythology, and picking and choosing which era of toys to add to the mix.
2008-2009: Transformers Animated
Yet another reboot, and one that deages Optimus Prime (as much as one can de-age a robot) into something approaching a teenager, making him the leader of five Autobots stranded on Earth, fighting Decepticons and, oddly enough, human supervillains in a future Detroit.
And don't forget...
Alternators - A toyline where the robots turned into accurate cars licensed from real world car manufacturers.
Robot Masters - A Japanese toyline accompanying Energon and Cybertron, but set in G1 continuity.
Star Wars Transformers/Marvel Transformers/Disney Transformers - A toyline where robots turn into licensed characters or vehicles from other franchises.