From the Linux distributions Wikipedia entry:
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions (often called distros for short) are Operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications.
What distinguishes them is the hardware they supposrt, packaging, kernel patches, what set and versions of applications they ship, their documentation, install methods etc. Other "classifications" are whether they are more oriented towards end users or servers.
Some of the distributions (Debian, Gentoo, Fedora and others) are used as a "starting point" for other distributions (Ubuntu is derived from Debian for instance). That means that the creators of for instance Sabayon Linux used a Gentoo distribution to start their development effort, and keep track of Gentoo's evolution to some extent.
You can look at the Distrowatch search page for this kind of examples.
"RPM-based" distributions is a different classification. RPM is a package management system, not a distribution. Some distributions use it (RedHat and Suse comes to mind) directly or via one of its frontends. Others use different systems (pacman for Arch, portage for Gentoo). The package management system is one of the important differences between distributions.
Regarding versions, there are no strict criteria. The distribution developers/managers decide on what versions/patches/new software they want to include in a new version, polish it, and when it's ready, they ship it. There isn't a consistent versioning scheme across distributions.
For your last question I'm not sure I understand, but you could say that Windows NT, 2000, XP, 2003/Vista, 2008 and Windows 7 are "versions" of the Windows "distribution". And they are all in the Windows NT family of Windows releases.
So if you want to draw a parallel with Linux distributions, yes, each windows "release" is closer to a version of a Linux distribution. And the "Windows NT" lineage is equivalent to the RedHat or Suse lineage for instance.
(One of the similarities of these "lineages" is that there usually is a major revision of the kernel between Windows releases, and that's also the case for a lot of Linux distros.)