OS == Operating System. The base software on your computer. This is what is used to interface with all the hardware your computer uses. Keyboards, mice, printers, video cards, sound cards, network cards and similar. The OS is also the foundation on which your other software programs run. Such as your browser (Firefox, Opera, Internet Explorer), word, spreadsheet, and other office type programs. Programs use the OS to communicate with the hardware.
Windows based Operating Systems:
Windows 95, 98, Me, 2000, XP, Vista
Linux based Operating Systems:
Suse, Red Hat, Ubuntu, and many more
BSD based Operating Systems:
FreeBSD, NetNSD, OpenBSD, Darwin
Apple Based Operating Systems:
System Software, Next, Mac OS
Ther are more types of operating systems out there. Risc, Dos, and OS/2 are some of the older favorites.