SVN is Subversion and it is a source control system. It is a PHP web based tool to administer an Apache Subversion repository server. It allows PHP scripts to communicate with SVN repositories and working copies without direct command line calls to the svn executable. If you are not currently using some other source control system; you are wise to invest in one. SVN is rather easy to use, compared to some other open source control systems, and has some neat tools on top of the source repository to make things easier for you.