The seated calf raise is an isolated strength exercise for the calf muscles. it can be done with a dedicated machine, or with a barbell.
The calves has two major muscles: the gastrocnemius and the soleus. A seated calf raise works on both of these muscles, but mostly on the soleus which is the inner calf muscle, underneath the gastrocnemius. To guys looking for bulk, this may seem like a negative. But in fact, since the soleus lies under the gastroc, by thickening the soleus, you will cause your gastroc to "pop out" more.