They usually maintain the essentials such as operating systems, business applications, security tools, web-servers, email, PCs, local and wide area networking both hardware and software and mid-range server hardware.
System admins are usually charged with installing, supporting, and maintaining servers or other computer systems, and planning for and responding to service outages and other problems.
Other duties may include scripting or light programming, project management for systems-related projects.
In charge of the computer systems all the parts that make a computer function.