TOS is the operating system and GUI package that is designed for Atari's ST and TT lineup of computers. The ST and TT, for those that don't know (and you probably don't), are computers designed with a Motorola 68k processor (similar to the early Macintosh). First produced in 1985, the ST was similar to the Commodore 64 in that it built everything into the keyboard. While I'm not sure, the TT appears to be more like a traditional desktop computer.
TOS consists of two parts: "Total* Operating System", which is a customized version of CP/M, and "Graphics Environment Manager" (GEM), which is the GUI that runs on top of TOS, and shares many common features with the initial DOS release (but not the later releases). Both TOS and GEM are stored entirely in the system ROM.
This is what the desktop looks like.
If you're wondering why the fonts and icons look so huge, this is actually in Low resolution, which is 320*200. This is obviously meant for viewing on some sort of composite display. However, while this is running at 320*200, the emulator I used is scaling it up to 640*400 for viewability's sake, and since the screenshots would be way too small to see anything if I shrunk them to normal size I just left them as is.
The desktop consists of a few icons, with a common title bar. Instead of showing drives, for each drive TOS shows a "drawer" icon, each of which are labeled A, B, C and so on, similar to how CP/M and DOS label their drives.
Since GEM for TOS wasn't affected by the Apple Computer v Digital Research lawsuit, the windows in this version remained Mac-like and can be resized infinitely. The windows are also permitted to overlap.