Suppose you want to apply revision control on files in a directory called ProjectFoo. In that case you need to set up a repo (short for repository) in ProjectFoo directory, which is referred to as the working directory of the repo. The repository is the database where the meta-data about the revision history are stored. For example, Git uses a hidden folder named .git inside the working directory.

You can have multiple repos in your computer, each repo revision-controlling files of a different working directly, for examples, files of different projects.