First introduced in the Transformers animated movie, Galvatron was created by the world-eating Unicron as his agent to help destroy the Autobot Matrix of Leadership. Rebuilt by Unicron using the body and personality of former Decepticon leader Megatron, Galvatron shared many of the same personality traits and also came to lead the Decepticons, but managed to take them to even further extremes.
In essence, Galvatron was part of the big move to kill off a lot of characters that kids knew and loved (and thus, theoretically already owned) and replace them with a new group of characters that they would hopefully become attached to enough to pick up again. This worked to an extent, though later even Hasbro admitted that killing off so many franchise characters was probably not the best move. For his part in this “master plan”, Galvatron was about as strong a villain as Megatron.
A bit more unpredictable than the original Decepticon leader, it would be pretty fair to call Galvatron literally insane. His charming penchant for shooting his own troops often led to the unsuccessful end of his own plans. Also, he literally destroyed two planets, which is pretty crazy. Add to the fact that he was voiced by Leonard Nimoy (at least in the movie), and Galvatron ended up being a pretty good replacement for Megatron.