Git is free software that allows a group of users to work on a project simultaneously in the form of a comprehensive version management system.
Files are not stored on a central server, as is the case with Subversion, for example, but each user has a copy of the current repository. The synchronization among each other can be done in different ways.
A further advantage is the cryptographic protection of the project history, so that any changes can always be traced unaltered.
Gitlab website: gitlab.tu-clausthal.de