The somatic nervous system is the part of the nervous system that controls voluntary functions. In other words, it controls functions that the brain or a sensory input gives a specific neural command for as part of a sentient thought process. In particular, the SNS controls muscle contraction of the skeletal muscles in response to voluntary thought. When you think to raise your arm in the air, the SNS sends a command to your arm muscles to contract to make the action happen.
The autonomic nervous system, on the other hand, controls involuntary functions. In other words, it controls functions that an organism does not have a specific, conscious thought for. These functions include breathing, heartbeat, digestion, sweating, etc. When you eat, you do not have to specifically think about making your stomach muscles work to digest and the move the food through your body. These functions happen automatically. The ANS is housed in the medulla oblongata of the brain.