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Git is a program that helps multiple developers work together on a program simultaneously. Every revision to the source code is retained, so if something goes wrong it is always possible to go back to a previous state.
You can probably skip this if you have never worked with Subversion or CVS. In Subversion you check out a working copy, typically trunk, typically only the last revision (HEAD). With Git you clone the full repository, all branches, entire history. But the repository remembers where it came from: the origin, where git pull and git push will refer to. Subversion identifies revisions with IDs of decimal numbers growing monotonically which are typically small. That is impractical in distributed systems like Git. Git identifies revisions with SHA1 IDs, which are long 160-bit numbers written in hexadecimal. It may look scary at first, but in practice it is not a big hurdle - you can refer to the latest revision by HEAD, its parent as HEAD^ and its parent as HEAD^^ = HEAD~2. Cut'n'paste helps a lot and you can write only the few leading digits of a revision as long as it is unique, Git will guess the rest. In Git you cannot pull (update) or merge or branch or switch branches with uncommitted modifications. When you get used to the Git way this restriction starts to make a lot of sense. But sometimes you want to pull and do not want to commit, in this case you can @git stash@ your modifications and “unstash” (@git stash pop@) them after you have updated. @git commit@ and @git push@ are not the same thing. You commit locally (@git commit@) and then you publish your changes to origin (@git push@) or not. You can @git commit@ several times before you @git push@. A @svn commit@ corresponds to @git commit@ with immediate @git push@. Git does not track directories or files, only content. So you cannot @git add@ and commit an empty directory. @git checkout@ is at first sight rather different from @svn checkout@. You can use @git checkout@ to switch to another branch or to revert modifications but not to clone the repository. However, the more you use Git the more @git checkout@ will resemble @svn checkout@.