The word DOS can be used to describe many Operating Systems : PC-DOS, MS-DOS, DR-DOS, Novell DOS, FreeDOS, SuperDOS, and many, many others.

DOS was the first operating system for the PC when IBM selected it from Microsoft (which had licensed it from Seattle Computer Products, which called it QDOS for Quick and Dirty Operating System). Since then many companies have cloned it and since Microsoft has stopped development advanced versions of DOS can be gotten from other companies.

My personal favorite version is Caldera DR-DOS, which is at version 7.02. It can coexist with Windows 95 and 98 and Windows 3.x can be installed on it. The system requirements are so low all you have to have is 640K of RAM and a hard disk with 6 MB (basic installation) or 12 MB (full installation) free space. If you have at least a 286 and 1 MB of RAM you can have DPMS support .If you have at least a 386 or above CPU and a 4 MB of RAM you can multitask (something most DOSes don't have).

There are also many graphical enhancements to DOS including GeoWorks, Desqview, and Microsoft Windows (except for NT). They often come with their own programs, With that have higher system requirements. Windows 95 and 98 require that you have a 486/66 with 8 MB RAM even though that should say Pentium 133 with 16 MB of RAM.