Probably everything you need to know about Extras, Extras-testing and Extras-devel: http://wiki.maemo.org/Extras

About Garage, it is true that is not well documented at least to find out what it does and how it relates to the rest of maemo.org. See http://wiki.maemo.org/Task:Using_garage.maemo.org - I have added this task back to the proposals for maemo.org sprints.