Sympathetic Nervous System
Think about a time you've been in a really dangerous or frightening situation. In that situation your body responded by, for example, increasing your heart rate, raising your respiration, moving blood from one place (e.g., your digestive system) to another (your muscles), etc. These responses are all "arousal" responses which get you ready to fight or flee in times of danger. It is the sympathetic nervous system that is responsible for these responses that get your body aroused to respond. This system works with the parasympathetic nervous system which is responsible for then calming your body after the arousal (to get you back to normal). Both of these are actually part (subparts) of the autonomic nervous system.