Subversion is a tool for revision control. Possibly most people know one of the main features of revision control. It is implemented in most editing softwares. You can find it in the (however you call it): history-or-undo-redo-functions. If you recognize that you did some shit editing e.g. your MS-Word-Document you can step back to the version before the shit happened using the undo-function. Subversion is a tool doing exactly this with some more features:
- you never loose your previous version. (e.g. in the MS-Office-Tools you loose your history when you close the file)
- one version can include multiple files
- support for a multi-user-environment working on the same versions
- you can easily reload a specific version
- you have integration in different IDEs
A detailed description can be found in wikipedia.
In my company we are using subversion for nearly two years now. It was a choice between CVS and SVN because both were open source and free and our external partner for software development used CVS. 'Cause SVN was said to be the successor of CVS and it removed some restrictions of the CVS we decided to start with SVN. In the meantime our partner also migrated to SVN.