CVS is the Concurrent Versions System. CVS is a software configuration management (SCM) tool. It allows developers to collaborate on projects transparently across networks and client platforms.
It works best on plain text files, but can also handle binary files. Some functionality will be lost on binary files as CVS can not perform the same operations on these files. For instance, CVS has no meaningful way of displaying the difference between two images.
A replacement for CVS, Subversion 1.0, was released in 2004. Another interesting SCM tool is OpenCM which still has some way to travel.
The OpenBSD team started developing their own CVS-compatible SCM tool focused on security which in time hopefully encourages the Subversion developers to do better.