The Autobots
The Autobots are the primary enemies of the Decepticons. The Autobots fight for the
side of good. Powered by the Autobot Matrix of Leadership they fight the Decepticons for
control of Earth and Cybertron, their home world. Optimus Prime, a transforming truck with
trailer, is the leader of the Autobots. 
They leave Cybertron in order to find new sources of
Energon, the main power source on Cybertron. Along the voyage, the Autobot ship is
attacked by the Decepticons and crashes lands on Earth.
Four million years later they are awaken by activity volcano they crashed into. The year is now 1984, when the series originated in America/Canada. When the landed on Earth they had to stop the Decepticons from pulling Cybertron into Earth atmosphere. After they succeeded, the Autobots struck an alliance with Earth and all it citizens.
In 1986, when the first Transformers movie is released (Transformers: The
Movie) Optimus is killed in battle with Megatron (leader of the Decepticons.) Before his
death, Optimus passes the Autobot Matrix of Leadership to Ultra Magnus, telling him that
it will "Light their darkest hour." As the movie progresses both the Decepticons
and Autobots must destroy Unicorn, a giant-planet eating robot. In the final battle Hot
Rod, an Autobot solider, opens the Autobot Matrix of Leadership to become Rodimus Primea
and uses its power to destroy Unicorn.
Rodimus Prime goes on to lead the Autobots in a victory over the Decepticons and start a new "Golden Age" on Cybertron.