GEOS can refer to one of several operating systems, all of which stand for "Graphical Environment Operating System." It was created by Berkeley Softworks, which later renamed itself to GeoWorks.
The most common is GEOS for the Commodore 64. In 8-bit form, GEOS was also ported to the Commodore 128 (called GEOS 128) and the Apple II. These 8-bit operating systems are 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.
GEOS was also released in a 16-bit form for IBM-compatible PCs, called GeoWorks, and this form uses multitasking in a Motif-like windowing environment.
Shortly after the end of Commodore, GEOS and GEOS 128 were sold to Creative Micro Designs, then it was passed to one of the most prolific GEOS programmers, Maurice Randall and Click Here Software (AKA CmdrKey.com). In 2004, Randall made GEOS and GEOS 128 free for download, and he reduced the price of boxed disks to $19.95 each. [As of 2012-03-30, links to purchase GEOS were broken --ArielMT]