Autonomic nervous system:
part of the peripheral nervous system, the autonomic nervous system is responsible for many "automatic" functions like control of heart rate, respiration, digestion, sexual arousal, etc. These are things we don't have to--and sometimes cannot--exert conscious control over. The counterpart of the autonomic nervous system is the somatic nervous system, which is involved in voluntary actions that we are aware of.
Watch this 2-Minute Neuroscience video to learn more about the divisions of the nervous system.