All Mountain Bike Parts Descriptions

#4) Brake Levers - Hand levers to stop the bike.
  The right lever controls the rear brake, the left controls the front brake, unless you have gotten used to a motorcycle brake style, which is reversed, this, you should know when you jump on someone's bike and try to do a big skid. If you hit the wrong brake lever, you may throw yourself over the bars onto the ground. I've seen it before.
  Most levers pull a cable to control the brakes, but hydraulic levers like on a motorcycle are becoming standard on many of the more aggressive style bikes.

Bike Parts Diagram