Great arms mean toned and defined biceps and triceps. Both muscles work as a pair to flex and extend the elbow. The biceps flexes the elbow, while the triceps extends the elbow. These muscles should always get the same attention to keep the body balanced. In other words, don't focus only on one side of the arm; train both for optimum results.
A unique feature of both muscles is that they both have different "heads." This simply means that one muscle has more than one origin. For example, the biceps, one muscle, has two different heads or origins, and one insertion point (place where the muscle inserts into a tendon and attaches to bone). The triceps has three different heads or origins, with one insertion point as well.
Even though there is more than one origin, the muscle is still considered only one muscle. To get the best results from training the arms, simply change your hand position during a biceps or triceps exercise. Using this technique, you can emphasize one or more of the heads of the muscle, ultimately getting the most definition in each of the muscles.