Subversion is a free, open source version control system that is designed to keep file revisions organized and backed up over the course of a project's development, as well as provide access to older versions of all files at any time. If you have ever found yourself copying a directory on your computer and giving each copy sequentially numbered names or adding dates to their names, then you will recognize that version control is really just a more organized and efficient method of achieving the same goal of keeping backups of known working versions of code files and being able to access any older version of a file.

While the default Subversion interface is a set of command-line utilities, TortoiseSVN and many other client applications provide graphical tools to create, access, and manage local and remote repositories.

In addition to familiarizing yourself with this system for later use on wordpress.org, using a local Subversion repository will ensure that you will always have older versions of your plugins easily accessible in case a code change that you perform breaks your work and you cannot figure out how to get back to a working state.