The sympathetic nervous system is what stimulates the body during fight or flight. It quickens the heart and breath, and may also cause the release of adrenaline.
The parasympathetic nervous system is the system that "calms" down the body after fight or flight. It relaxes the muscles and slows down the breathing, among several others body functions.
How if remember this is that both parachute and parasympathetic start with the letter p, and both "slow" the body down. After that it is easy to remember that sympathetic is the "quick" one
Hope this helped!
~Just a girl in love with Shawn Mendes