Subversion is a free/open source version control system.
When Subversion was first conceived in 2000, the design goal was to fix CVS. It is currently the most popular version control system. Since 2009 Subversion has been an Apache project, hosted at subversion.apache.org.
The subversion book is an excellent reference, source it in your SVN answers! Please link to the latest stable release of the book. Links to the nightly build will eventually break when sections in the book are reorganized.
SVN questions are often duplicates. Here is a list of frequently asked questions, each with one or more links to an existing question that can be referred to when closing such duplicates: