The autonomic nervous system together with the somatic nervous system forms the nevous system of vertebrates.
The automony in the name references, that this system is mostly uncontrolled by the concious brain. This differentiation is not absolute, as that is also true for parts of the somatic system (for example motor reflexes).
The autonomic nervous system controls essential life functions = vital functions, like respiration, digestion and metabolism.
Single organs and organ-systems (like the sexual organs, of the inner muscles of the eye) are also influenced by the autonomic NS.
The autonomic NS is divided into parasympathetic and sympathetic and intramural systems. The parasympathetic and sympathetic systems have atagonistic effects of their influenced organs.