An operating system (usually abbreviated "OS") is software that manages the resources of a system. The OS allocates memory, manages contact with hardware and peripherals, provides file system functions, and handles the user interface.
Wikipedia has a thorough discussion of operating systems, including history, types of operating systems, and numerous examples (some of which may not be listed here).
The Operating System Documentation Project has articles discussing numerous operating systems, along with many technology companies (as they relate to OS's).
The pages in this category contain information about different operating systems. Some tasks may have been solved for specific operating systems as well.