There was a story about 2 brothers who had a natural talent for alchemy called Fullmetal Alchemist. This story was originally told in a manga, but before the manga was finished an anime was put out. The anime had to change and create some original story elements because the manga was still ongoing at the time.

After the manga had finished they wanted to revisit the anime with a version closer to the actual story told in the manga. This would be known as Fullmetal Alchemist: Brotherhood. The only thing was due to retreading some story elements told in the original anime, they shortened some story beats for FMAB.

This is vastly different than the original cut. We have a new opening to recap the ending of Part 1 and a subsequent scene not seen in Part 1. The tone is less cartoonish and more balanced. There is a lot of new music and deleted material used to flesh out the movie. Much like Part 1 this has the focus on Peter and Gwen's relationship without all the other distractions.

Electro is pushed back to a secondary villain and Harry is brought up to being the primary villain. Electro's motivation is different as well. The accident simply drove him insane and he hears voices in his head.