This nice article describes how to update Ruby Gems on Mac OS X Leopard 10.5:

Updating_to_RubyGems_1.3.0_on_Mac_OS_X_Leopard_10.5.4

Git Cloning and Git Forking differences are described in this article

Git Cloning

Cloning is to make a copy of a remote repository into your local development environment.

Git Forking

Forking is to clone an entire remote repository into another remote repository.

I thought this article about setting up a new Rails app with Git provided a good way to think about Git repositories:

Git repository’s are a-dime-a-dozen. There’s no such thing as a working copy, everything’s just a git repository containing copies of other git repositories. When you develop locally you’re committing to a local git repository, stored in the .git directory of your project.