AB starts at level 20, bracket size is 20-29, so on and so forth.
This is the loading screen for Arathi Basin.
The goal is to control locations on the map ("Resource Nodes"). While a node is controlled, it generates resources for your side, and the first side to gain 2,000 resources wins the contest. Each side has up to 15 players. There are five Resource Nodes on the map: the Farm, the Stables (ST), the Blacksmith (BS), the Lumber Mill (LM), and the Gold Mine (GM). A node only earns resources when it is captured.
This is the map for Arathi Basin.