Git is a version control system, whereas Gitlab/Github are source-code hosting applications. You use git to interact with Gitlab/Github. Without going too in-d

Git is a version control system, whereas Gitlab/Github are source-code hosting applications. You use git to interact with Gitlab/Github. Without going too in-depth, they allow teams to set up accounts on a central website, where team members can contribute to that central git repository. There are also other project-management features baked into the tool (code review, issue tracking, etc). Of course, this can all be done using a bare git server running on some publicly-available computer, but they offer nice UIs and very friendly onboarding so that makes them attractive for teams.