A Version Control System or VCS is a system that is capable of recording changes made to a file or a group of files over time. Git and Mercurial are two of the most popular version control systems. Important uses of a VCS are: Check what was the last modification that caused a problem Compare the changes made over time Identifying who introduced a new issue and at what time Revert a file or files to some earlier state Revert the complete project to a previous state