What is SVN?

• SVN is the abbreviation of Apache Subversion. It was named after the command name svn. 
• SVN is a software versioning and revision control system distributed under an open source license.
• SVN is used by developers (or project members) to maintain current and historical versions of files such as source code, web pages, and documentation. 
• SVN is the mostly known as the successor of widely used Concurrent Versions Systems (CVS), thus its goal is to be the most compatible successor of it. 
• SVN has been used widely by open source community. (e.g. Apache Software Foundation, Free Pascal, SourceForge, etc)
• Google Code (Google Official Developer Site) and Microsoft CodePlex (Microsoft Open Source Project Hosting Site) both offers subversion hosting for their open source projects.

Graphical Subversion Clients

If you want to use a graphical Subversion client, below are some known options:

TortoiseSVN - an open source Apache Subversion (SVN) client, implemented as a windows shell extension (That means it is integrated into the Windows explorer). It's intuitive and easy to use, since it doesn't require the Subversion command line client to run. Simply the coolest interface to (Sub)Version Control.

Versions - a Mac only, commercial client. Offers the best way to work with Subversion on the Mac. 

RapidSVN - a multi-platform GUI front-end for the Subversion revision system. (note: is hosted by the company who created subversion)

svnx- a Mac only, open source client.