The sympathetic nervous system is in charge of "fight or flight" while the parasympathetic nervous system is in charge of "rest and digest"
sympathetic nervous system:
The sympathetic nervous system (SNS) is part of the autonomic nervous system (ANS), which also includes the parasympathetic nervous system (PNS). The sympathetic nervous system activates what is often termed the fight or flight response.
parasympathetic nervous system:
The parasympathetic nervous system (PSNS) is one of the two divisions of the autonomic nervous system (a division of the peripheral nervous system (PNS)), the other being the sympathetic nervous system.