GASTROCNEMIUS

Origin:  Medial and lateral epicondyles of the femur

Insertion:  Calcaneal tuberosity via the Achilles tendon

Innervation: Tibial nerve (S1, S2)

Action:  Ankle (talocrural joint) - plantar flexion
      Knee - flexion

SOLEUS

Origin:  Posterior surface of the head and neck of the fibula and the soleal line of the tibia via a tendinous arch

Insertion:  Calcaneal tuberosity via the Achilles tendon

Innervation: Tibial nerve (S1, S2)

Action:  Ankle (talocrural joint) - plantar flexion

What does that mean?  The gastrocnemius is the muscle with two heads, and is the one that people like to flex and show off.  It crosses both your knee and ankle joint, and is best strengthened with the knee straight.  When you bend the knee, you put the gastrocs on slack, and at this length/tension relationship, the gastrocs are not very effective at plantar flexing the ankle.  
The soleus is a flat muscle (google sole fish) that lays under the two heads of the gastrocs.  Just because you don’t see this muscle when you’re flexing, doesn’t mean it’s not important.  The soleus helps give your gastrocs some bulk, and helps them pop.  Strengthen your soleus by doing resisted ankle plantar flexion with your knee bent.
Collectively, the gastrocs and soleus are known as the triceps surae.  They both have a distal tendinous junction at the Achilles tendon.  Both muscles are extremely important in gait and dynamic activities, and both become ineffective with an Achilles tear.