The Full form of DOS is Disk Operating System. The term DOS can refer to any operating system, but it is most often used as shorthand for MS-DOS (Microsoft Disk Operating System). It was originally developed for IBM by Microsoft; MS-DOS was the standard operating system for IBM-compatible PCs (personal computers). DOS initial versions were very simple and resembled CP/M (another operating system).
Later on, the versions came are not very simple and became increasingly sophisticated as they have incorporated the features of minicomputer operating systems. However, DOS is still a 16-bit operating system and does not support multitasking or multiple users. It has been widely acknowledged for some time that DOS is insufficient for modern computer applications. Microsoft Windows helped alleviate some problems, but still, it sat on top of DOS and relied on DOS for many services.
Even Windows 95 sat on top of DOS. Although, newer operating systems such as OS/2 Warp and Windows NT can execute DOS-based programs, but do not rely on DOS to the same extent. It is expected that DOS will eventually disappear as these newer operating systems gain market share. In the meantime, Caldera, Inc. markets DR-OpenDOS (a version of DOS) that extends MS-DOS in significant ways.