This article is from the Apple II Programming FAQ, by Jeff Hurlburt with numerous contributions by others.
The GS can support several. Some of the more popular OS environments include ... DOS 3.3- usually Beagle's Prontodos or some other speeded version of the original DOS 3.3. This is the 5.25" diskette-based disk operating system used for years on earlier Apple II's. It's commands are designed for use from BASIC programs or from the keyboard. Many old Apple II games and other wares are on 5.25" diskettes which boot DOS 3.3. ProDOS 8- e.g. ProDOS v2.0.3. This is a disk operating system which supports a variety of devices (e.g. 3.5" drives, hard drives, etc.) and allows sub-directories. Commands are very similar to DOS 3.3 and are designed for use in BASIC programs or from the keyboard. BASIC and 'system' programs can also utilize direct CALLs to well-defined ProDOS Machine Language Interface routines. Apple Pascal- This is an early 1980's implementation of UCSD Pascal which can run on 40-column and 80-column Apple II's. System- Originally called "ProDOS 16" and later "GS/OS", the collection of 'system stuff' (which includes GS/OS) is today called "System". The System Finder utilizes Toolbox routines to supply a super-res desktop, Windows-like environment. Both ProDOS 8 programs and 'GS Applications' (i.e. wares that need GS/OS to run) can be started from the desktop. Depending upon factors such as installed RAM and presence of a hard disk, System will usually be System 5.0.4 or System 6.0.1. System 5.0.4 is generally appropriate for 1.25MB or smaller non-hard disk GS's. Booting a reduced System 6.0.1 often works fine for running Sys 6 software, too. System 6.0.1 is definitely recommended as the default System for larger memory GS's with a hard disk. --Rubywand and David Empson GNO- GNO is a UNIX-like multitasking environment for the Apple IIgs which runs on top of the native operating system, GS/OS. --Rubywand, David Empson, Devin Reade, Jeffrey Iverson