GEOS, short for Graphic Environment Operating System, is an 8-bit GUI released in 1986 for the Commodore 64. By default it came bundled with two applications: GeoPaint and GeoWrite.
At one point it was the third most popular OS, behind MS-DOS and the Mac OS of the time, and more applications were available from both Berkeley Softworks (creator of GEOS), as well as other third parties. More versions of GEOS for the C128, the Apple II, and even the Plus/4 were also released.

Unlike alot of proprietary software of the time, GEOS could take advantage of expansions such as RAM add-ons and mice.

This is the about screen. Not sure which version this is.

The desktop is not unlike that of the Mac OS of the time. You have an always-present menu at the top of the screen displaying options that are applicable to the currently open window. On the desktop, there are a list of disk drives as well as a "wastebin", aligned to the right like the Macintosh.
Unlike the Mac, there is an icon for every installed printer on the desktop, and there is also an always-present window displayed of the main system disk.

Also unlike early Macs there are no scroll bars. Instead there is what looks like a curled-up page corner in the lower left, and this functions as a sort of "page" and allows you to flip through various "pages".

Interestingly there is an open option under both the Disk and File menus. The file menu opens the selected file, and the disk menu opens the selected disk.
Each menu also contains options specific to that type of object. Like the Macintosh there is no Copy option, rather there is a Duplicate option.

There are all sorts of extra properites stored with the file, including the author and some comments. While still limited, the file name restriction was far less than what was available on other platforms.