Version control software allows users to track changes made to files over time by assigning each set of changes a revision number. Subversion is a free, open source version control system where data is stored in a central repository. Users check out a working copy of files from the repository onto their local machine where they can make edits. Changes are committed back to the repository to create a new revision, while updating synchronizes the local copy with any changes others have committed. Conflicts may occur if multiple people edit the same file.