That probably depends on who you work for, but typically they are in charge of your network infrastructure and hardware as well as PC's, software, printers, etc. Pretty much anything that uses or connects to the network, the admin has to keep tabs on and maintain. If you are under a corporate umbrella, those responsibilities may be drastically different as they may have corporate-wide standards for many things like hardware, software, and they may also not want local admins doing configuration of routers, switches, servers etc.