Rated battleground (RBG) is a way to increase PvP rating and get conqueror points. Once a week each of RBGs allows players to get bonus points, completing daily quests. The points can be used to get epic rewards and equipment which is valuable in PvP fights. If you win you get higher rating, honor and conqueror points. The higher your rating and result on RBG the more weekly points you can spend and the better items you can buy.
Here you can buy RBG Rating Boost or Hero of the Alliance / Horde for EU or US Servers.
You'll get guide for completing RBG Rating
You can fight a battle in RBG or Arena after reaching max level. You win a battle and receive scores - rating, you can rise your Arena rating in the battles with other players. Conquest is a currency that gives you an opportunity to get best gear for PvP. PvE gear is usable in PvP as well as PvP gear is usable in PvE. Your goal is win bg of opposite team.
Combatant (1400 - 1599): wrist and waist. Title and achievement
Challenger (1600 - 1799): hands, legs, feet. Title and achievement (12% of people by WoWHead stats)
Rival (1800 - 2099): head, shoulder, chest. Title and achievement (only 5% of people by WoWHead stats)
Duelist (2100 - 2399): cloak, weapon enchant. Title and achievement (only 2% of people by WoWHead stats)
Saddle mount progression
Elite title and achievement(2400+): tabard.
Active Game time
Max level of character