A virtualization administrator is an employee whose responsibilities include virtual environment set up and maintenance, in addition to traditional sysadmin duties. That virtual environment could consist of operating systems, network resources, servers, storage, desktops, applications and/or data.
Server virtualization makes each virtual server look and act like a physical server, multiplying the capacity of every single physical machine. The concept of server virtualization is widely applied in IT infrastructure as a way of minimizing costs by increasing the utilization of existing resources.
In OS virtualization, the operating system is altered so that it operates like several different, individual systems. The virtualized environment accepts commands from different users running different applications on the same machine. The users and their requests are handled separately by the virtualized operating system.