Below are a few links that cover the history of the Transformers franchise over the last 25 years.
Many Looks of Optimus Prime
With the new movie giving a new generation some Optimus Prime love, click the link for a prime on the many versions of Prime through the various toy lines. Thanks to Brian for the link.
Started in 1980 with Takara Toys called Diaclone, expanded in 1982, introduced to Hasbro in 1983, who turned over Transformers universe building chores to comics writer Bob Budiansky and then launched the line 1984. More details at the link.
Transformers Lines Through the Years
A nice simple breakdown of the various Transformers toy lines over the last 25 years so now will now what "G1" or Generation One means, Beast Wars, Armada, Energon, and more.
Covers the basics of the where they are from, why are on earth, and why transform.