Jessamyn West (Open Library) wrote a post that eloquently explains git, GitHub, and how they empower users to work on open source projects:
In the past we’ve always said that Open Source was great because if you didn’t like something you could change it. However it’s only been recently that the tools to do this sort of thing have become graspable by the average non-coder. I am not a coder… So, I “forked” this code (i.e. got my own copy) and opened it up to see if I could see where it was doing its thing and if I could change it to make it do something slightly different.