Barret is the strongman of your party. Most of his gun attacks are long range, so you will want to keep him in the back. Unlike strongmen in most RPGs, Barret can actually use magic and materia. This makes Barret capable of damaging any enemy, including those annoying bosses that are too far away for standard weapons to reach. A friend of mine made a hilarious observation on Barret: on the Shinra boat heading to Costa del Sol, Barret is in a Sailor getup. Doesn't he look just like a black Stay-puff Marshmellow man? You know, the big white guy from Ghostbusters? I think so too!
Barret's home town is Corel. Corel was destroyed a few years ago, by a mako incident that Shinra was trying to cover up. Shinra set the village ablaze. Barret and one of his friends was just returning when it happened, and were attacked by Shinra soldiers. Barret was injured in his right hand, and his friend was sent tumbling down a cliff after being hit by Shinra bullets. Barret was never able to use his right hand again, therefor he had a gun grafted to it.
Barret is one of the terrorists Cloud is fighting for at the beginning of the game. He is a respected member of AVALANCHE, the terrorist group seeking to destroy Shinra's mako reactors. They claim they are doing this because Mako reactors suck up the planets life energy, but it is impossible to deny the horrer of Barret's past, and his hatred for Shinra, inc.