Both Windows and Linux come in many flavors. All the flavors of Windows come from Microsoft, the various distributions of Linux come from different companies (i.e. Linspire, Red Hat, SuSE, Ubuntu, Xandros, Knoppix, Slackware, Lycoris, etc. ).
Windows has two main lines. The older flavors are referred to as "Win9x" and consist of Windows 95, 98, 98SE and Me. The newer flavors are referred to as "NT class" and consist of Windows NT3, NT4, 2000, XP and Vista. Going back in time, Windows 3.x preceded Windows 95 by a few years. And before that, there were earlier versons of Windows, but they were not popular. Microsoft no longer supports Windows NT3, NT4, all the 9x versions and of course anything older. Support for Windows 2000 is partial (as of April 2007).
The flavors of Linux are referred to as distributions (often shortened to "distros"). All the Linux distributions released around the same time frame will use the same kernel (the guts of the Operating System). They differ in the add-on software provided, GUI, install process, price, documentation and technical support. Both Linux and Windows come in desktop and server editions.
There may be too many distributions of Linux, it's possible that this is hurting Linux in the marketplace. It could be that the lack of a Linux distro from a major computer company is also hurting it in the marketplace. IBM is a big Linux backer but does not have their own branded distribution. Currently there seem to be many nice things said about the Ubuntu distribution.
Linux is customizable in a way that Windows is not. For one, the user interface, while similar in concept, varies in detail from distribution to distribution. For example, the task bar may default to being on the top or the bottom. Also, there are many special purpose versions of Linux above and beyond the full blown distributions described above. For example, NASLite is a version of Linux that runs off a single floppy disk (since revised to also boot from a CD) and converts an old computer into a file server. This ultra small edition of Linux is capable of networking, file sharing and being a web server.