There are three primary muscles in the calf; the gastrocnemius, the bulging muscle you feel on the outside. This is worked best with the leg straight (standing calf press, leg press, sled).
The soleus, which runs underneath the gastroc. This is worked best with the knee bent, such as seated calf press. It is important to train this both for function, and developement of it makes the gastrocs more prominent.
The third is the much neglected Tibialis Anterior, the muscle that runs in front along the shins.
Check out exrx.net for excercise and anatomy descriptions for a better understanding.