Galvatron is the reincarnated form of Megatron.

Megatron fought to the death with Optimus Prime in Transformers the Movie. When the Decepticons were retreating in their shuttle (Astrotrain the triple-changer), Starscream--the second in command of the Decepticons, jettisons Megatron into space. It is then that Unicron, the mechanical planet that turns into a giant robot, finds Megatron and rebuilds him as Galvatron.

Galvatron is like his former self in many ways. He still has a giant arm cannon that can shoot mega-laser blasts. But Galvatron transforms into a giant laser cannon, instead of a hand gun like Megatron.

And Galvatron still has all the millitary strategy, ruthlessness, and cunning of Megatron. However, Galvatron is somewhat psychotic, and his tendency to act on his emotions rather than logic has caused many Decepticons defeats to be worse than need be (after all, Megatron knew when to back off).

Oh yeah, Gavatron is also the name of the Predacon leader in the Beast Wars cartoon. But he's nowhere nearly as cool as the real Galatron.