All Mountain Bike Parts Descriptions

#2) Handlebar Grips - Grips provide shock absorption and "grip" for your hands.
  They come in three sizes that I've seen. 3/4", 7/8" , and 1", but the 7/8" size or 22.2 mm. as it's also known, has become the standard. They are made from rubber, foam, cork and some have gel inserts. All of them are designed to absorb shock but still give a solid grip to the bars.

Bike Parts Diagram