You clone a repository with
git clone [url]
For example, if you want to clone the Stanford University Drupal Open Framework Git library called open_framework, you can do so like this:
$ git clone git://github.com/SU-SWS/open_framework.git
That creates a directory named open_framework (at your current local file system location), initializes a .git directory inside it, pulls down all the data for that repository, and checks out a working copy of the latest version. If you go into the newly created open_framework directory, you’ll see the project files in there, ready to be worked on or used. If you want to clone the repository into a directory named something other than open_framework, you can specify that as the next command-line option:
$ git clone git:github.com/SU-SWS/open_framework.git mynewtheme
That command does the same thing as the previous one, but the target directory is called mynewtheme.
Git has a number of different transfer protocols you can use. The previous example uses the git:// protocol, but you may also see http(s):// or user@server:/path.git, which uses the SSH transfer protocol.