The Concurrent Versions System (CVS), also known as the Concurrent Versioning System, implements a version control system: it keeps track of all work and all changes in a set of files, typically the implementation of a software project, and allows several (potentially widely separated) developers to collaborate. CVS has become popular in the open-source world. CVS is released under the GNU General Public License.

Source: http://en.wikipedia.org/wiki/Concurrent_Versions_System