All Mountain Bike Parts Descriptions

#5) Stems - Holds the handlebar in place.
  They support the rider's weight on the handlebars, so they are made sturdy with thick aluminum, titanium, steel, or carbon fiber.
  The range of lengths and rises (or drops), available with stems can adjust your handlebar to put your body into the perfect riding position. They come in Quill or A-head style The Quill style uses a wedge plug that tightens inside the forks steer tube. The A-head stem clamps around the forks steer tube and tightens the headset at the same time it is installed. Most better bikes are using the A-head style stems these days.

Bike Parts Diagram