Alright, so you have some kind of vague idea about what a Battleground is. Basically, Battlegrounds are where players go to consensually bash each others brains out. Or fry them. Or freeze them. These instances are designed for players to enjoy PvP, get a bit of honor and, if you're not already at max level, some experience. In the future, Blizzard has revealed plans to make the Battlegrounds even more like instances or dungeons by hinting at possible actual loot rewards.
