Previous to the war started by Megatron The Autobots were all the Transformers on Cybertron. After Megatron gathered together an army and named them Decepticons, the remaining Transformers were known as Autobots. In general, the term Autobot is used to refer to those Transformers allied to Optimus Prime and / or want to protect the AllSpark from Megatron and / or his forces (remember, Megatron is "dead' and there are sequels on the way.)
Optimus Prime claims that the name Autobot is derived from the English words Autonomous and Robot.
List of Movie Autobots
Optimus Prime
Ruler of Cybertron and leader of the Autobot forces on Earth.
Bumblebee
Autobot hero and scout.
Ironhide
Autobot weapon specialist.
Ratchet
Doctor and medic of the Autobots on Earth.
Jazz
Former second in command of the Autobot forces on Earth. Killed by Megatron.
Arcee
Female Transformer once a member of a squad lead by Bumblebee on Cybertron.