Operating systems (OSs) do many things: it is the block of software that runs in the memory of a computer which mediates between the hardware below and application(s) above; provides consistent access to various hardware for user programs such as applications and utilities; multiplexes hardware resources between two or more processes, programs, users; provides some common services to all software via an application programming (or protocol) interface (API); allocates memory and storage; schedules tasks.

On this page, OSs are arranged in three groups and levels: 1) Top group: issues spanning multiple unrelated OSs. 2) Middle group: types or classes of OS, or OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type, with its own category, and non-trivial, useful running code (more than a boot loader), and/or a body of worthwhile readings. It cannot be merely a new project with no significant code.