These points are taken directly from the Subversion project web site.
Subversion is meant to be a better CVS, so it has most of CVS's features. Generally, Subversion's interface to a particular feature is similar to CVS's, except where there's a compelling reason to do otherwise.
Do you need another version control system which is more scalable than Visual Source Safe? Are you a CVS user who finds the lack of directory versioning a difficulty? Are you paying too much for your current version control system? Do you not use a version control system right now?
If you answered yes to any of the above you need to look at the Subversion version control system.