An operating system is software "that manages the hardware and software resources of a computer" (W). A computer cannot run other software unless it has an operating system, although some programs (most often games) are packaged with their own lightweight operating systems sufficient to allow the game to run.
The process of loading an operating system into memory is called "booting"; this process must complete successfully before any other software can run. Booting requires a specially-prepared storage medium with a particular type of program in its boot sector; the medium can be anything which the computer's hardware knows how to read (based on instructions in its BIOS). For older computers, this usually means a floppy disk or the hard drive; newer computers (since about 1995) can "boot" from the CD-ROM and sometimes other devices such as network connections.