One version or revision of a product is the state where it is at a given moment in its development or modification. Version control is the management of various changes made on the elements of a product. A Version Control Systems allow us to manage different versions of each product developed and made possible customizations (eg for a specific client). In the case of software is used to track different versions of source code that are continually modified by the development team. The most used tool for this is Subversion.
Subversion is a software Version Control System that seeks to replace the popular CVS, which has several shortcomings. It is open source and is also known as svn (because this is the name of the command-line tool).