What is CVS?

In the field of software development, the Concurrent Versions System (CVS), also known as the Concurrent Versioning System, is a free software version control system. Version control system software keeps track of all work and all changes in a set of files, and allows several developers (potentially widely separated in space and/or time) to collaborate. Dick Grune developed CVS in the 1980s. CVS has become popular in the open source software world and is released under the GNU General Public License.

CVS is the most popular version control system available today.  It is robust and it is open source, so the software is free.