Apache Subversion (abbreviated SVN) is a free/open source version control system.
When Subversion was first conceived in 2000, the design goal was to fix CVS. It is now the most popular version control system. Since 2009, Subversion has been an Apache project, hosted at subversion.apache.org.
Apache Subversion has a GUI clients for Windows, Linux and OSX that provide better user-experience with source-control and make it much easier to understand the basic concepts of version-control for beginners.
Version Control with Subversion (SVNBook) (actual and updated)
- Subversion Version Control (PDF, 2005 and can be considered outdated)
The Subversion book is an excellent reference - source it in your SVN answers! Please use links (URLs) to the latest stable release of the book (links to the nightly builds will break when sections in the book are reorganized).
If you are new to Apache Subversion or version-control in general please take some time to read the SVNBook. The must-read articles are: