We use Git because it is an amazing structured collaboration process that enables thousands of people to work on projects without central coordination. There is a learning curve but it is worth the effort.
Git is like a central collaboration trunk with a million services that work on top of it.
The first one we use is Github: it's a hosting service for Git repositories.
Gitbooks is another service we use: it turns all the files you see on github.com/enspiral/handbook into the nice browsable, searchable, portable package you can see at handbook.enspiral.com.
Repo is short for repository: in Git, all the files for a project live in a repo.
A Github issue is a public discussion for work-in-progress.
So for instance, when I see something wrong with Loomio, I go to the Loomio repo on Github and say "I found a bug, this thing is broken!" Then we have a discussion about how to fix it. Sometime later someone can propose a fix, and reference the issue, so when the proposal is accepted, the issue gets closed.
In git lingo, the proposal is a pull request and it is accepted by merging in the changes.
Now that you have some Git skills, why don't you try proposing a change to this handbook.
If you have any questions that aren't answered by this guide, you can ask them by creating a new issue.
And a place to practice your new Github skills: