GEOS was created by Berkeley Softworks and was released for several home and personal computers, in two vastly different and incompatible editions. The 8-bit editions were released for the Commodore 64, 128, and Plus-4, as well as the Apple II-series. The 16-bit edition, typically called PC-GEOS to differentiate it from its 8-bit form, was released for IBM-compatible PCs.
PC-GEOS has been variously called GeoWorks, New Deal Office, and Breadbox Ensemble. In all versions and names, it's a multitasking environment relying on MS-DOS under the hood, presenting a Motif-like or Windows-like windowing environment.
Due to the limitations of the 8-bit computers it runs on, GEOS is neither multitasking nor multiuser; they can run only one program at a time. Even desk accessories, though accessible while using nearly any GEOS application, work by suspending the application in progress until the accessory quits.
Commodore adopted GEOS as its official OS for the Commodore 64 sometime around 1986 and bundled GEOS with its 64C for many years. Compute!'s Gazette, a Commodore-focused computer magazine, found in a 1987 survey that half its readership used GEOS.
Shortly after the end of Commodore, GEOS and GEOS 128 (the version for the Commodore 128) were sold to Creative Micro Designs (CMD) before eventually being passed on to a GEOS programmer named Maurice Randall. In 2004, Randall made GEOS free for download: http://cbmfiles.com/geos/index.php