The Gastrocnemius has two heads: the medial and the lateral. When fully developed, these two heads appear to form a diamond shape.
The Soleus is not visible when looking at the body from the outside as it lies underneath the Gastrocnemius on the rear of the lower leg.
The Soleus is most active when doing calf exercises where the knee is bent, such as seated calf raises.
The function of the Gastrocnemius is to elevate the heel (known as plantar flexion).
The function is the Soleus is exactly the same as the Gastrocnemius: to raise the heel. The only difference is that it works in a different position: with the knee bent.