Autonomous nervous system (ANS), is a division of the nervous system that controls our various involuntary movements like those of our heart muscle, our diaphragm (which compresses our chest when we breathe), our digestive tract, our glands (these secrete hormones, saliva, sweat, tears, stomach acid, etc.), etc.
Sympathetic nervous system (SNS), is a subdivision of the ANS that, along with the parasympathetic branch of the ANS (PNS) works to control our various involuntary movements. The SNS has basically the opposite actions on our involuntary movements as the PNS and is responsible for controlling breathing rate, the width of the airways (most drugs for asthma act on the SNS) and heart rate. The SNS is often called the “fight or flight” response, its actions in the ANS is usually on the more immediate functions of the ANS, like heart rate, breathing rate, etc.
Parasympathetic nervous system (PNS) is a subdivision of the ANS that is often called the “rest and digest” division, its actions are on housekeeping functions like controlling the secretions of glands, the movement of the various components of the digestive tract (hence PNS inhibitors are known to cause constipation), etc. Its chief neurotransmitter is acetylcholine.