The triceps brachii are divided into three parts, the lateral head, long head and medial head. The lateral head between your biceps and triceps, the long head is on the back of your arm and the medial head is located at your lower triceps down to your elbow. The purpose of the triceps is to extend the forearm; the long head extends and adducts the arm.
The triceps is a muscle which is usually neglected or trained in-effectively. Once again, the triceps are a bigger muscle than the biceps, but are still a small muscle so they should be trained with a less amount of sets compared to the larger muscles. As with biceps training, one should always add a compound movement along with an isolation movement. Close grip flat presses are great for building a foundation followed by and isolation movement such as triceps cable push downs which will hit all three heads.