Apache Subversion is a software versioning and revision control system distributed as open source under the Apache License. It is used to maintain current and historical versions of files such as source code, web pages and documentation.
Subversion (often abbreviated 'svn') is arguably the most well-known example of a centralized version control system. In a centralized system, there is generally considered one master repository that all developers pull and commit to. As such, frequent pulling and commiting as well as communication between developers is extremely important when working with svn.
Alternatives to Subversion
For downloads and documentation, visit Apache Subversion official site