Server OS is a type of operating system that is used to manage and control the resources of a server. The most common type of Server OS is UNIX, which is an open source operating system developed in the 1970s. UNIX is a multi-user, multitasking operating system that is used for a variety of tasks, including web hosting, file sharing, and database management. UNIX is also known for its stability and security, making it a popular choice for businesses and organizations.