All Mountain Bike Parts Descriptions

#20) Cables - The brake and gear transfer medium.
  Cables pull directly on the gear or brake device from the hand levers. They have a unique function as they flex with the frame or brake pivot action without losing tension. This means that the brakes won't fade or the gears won't shift on you.
  Hydraulic cables (used with hydraulic brakes and levers) use mineral oil or DOT fluid (like a motorcycle) instead of the standard steel braided wire, and they have the best braking performance.

Bike Parts Diagram