Version control, also known as revision control or source control, is the management of changes to documents, computer programs, large web sites, and other collections of information. Changes are usually identified by a number or letter code, termed the "revision number," "revision level," or simply "revision." Version control systems (VCS) most commonly run as stand-alone applications, but revision control is also embedded in various types of software such as word processors and spreadsheets, and in various content management systems. Revision control allows for the ability to revert a document to a previous revision. Git, a widely-used VCS, has its own topic; other popular systems include CVS, Subversion, Team Foundation Server (TFS), Visual SourceSafe (VSS) and Perforce.