Windows 98 and Windows Me are successors to Windows 95, each with slightly more features and hardware compatibility out of the box. (For instance, Windows 98 has native support for USB devices, while Windows 95 does not.) Windows 2000 is a completely different "breed" of Windows, which uses a completely new kernel system (derived from NT 4.0) and memory manager. The vast majority of programs written for Windows 9x will run in Windows 2000 without problems (barring games that use bizarre hardware addressing).
This was first published in November 2003