What do the triceps do?
The triceps runs down the back of the upper arm and makes up most of its size. It straightens the elbow, the movement in pushing yourself up from a chair. One of its three heads also crosses the shoulder, which is why exercises with the arm overhead work it a little differently.
How do you train your triceps?
Straightening the elbow against resistance trains the triceps. Every push-up and press works them, and moving your hands closer together shifts more of the work onto them. Overhead extensions add the long head, which is stretched when the arm is above your head.