Modern Operating Systems (most notably) consist of:
- security (device access, memory, operating system internal protections, processes access, cpu , probably more ...),
- process management & concurrency,
- memory management,
- file system & management,
- input/output management & device drivers,
- graphic user interfaces,
Different processess are responsible for these functions, they cooperate and communicate to achieve all these goals, and should provide simple interface to a user.