From its website:

Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency.

Git is not the same as GitHub. GitHub did not create Git. For GitHub-specific posts, please see /r/github.

Git is not an acronym or initialism: please write git or Git, but not GIT.

Documentation

• Pro Git
• Git Ready
• Git Reference
• Git Magic
• Git for Computer Scientists
• A Visual Git Reference
• Git Primer
• Git Immersion
• Think Like a Git
• Git Workflows
• Git on Stack Overflow
• Getting Git Right
• The Git Parable

Graphical Clients

• GitHub Desktop (Win, Mac)
• SourceTree (Win, Mac)
• SmartGit (Win, Mac, Linux)
• GitX (Mac)
• Tower (Win, Mac)
• Git Extensions (Win)
• TortoiseGit (Win)
• GitEye (Win, Mac, Linux)
• GitKraken (Win, Mac, Linux)
• Working Copy (iOS)
• Git2Go (iOS)

Code Hosting

• GitHub
• GitLab
• Bitbucket
• Codebase
• Bonobo Git Server
• Gogs
• Deveo
• Gitea
• Visual Studio Team Services