Backtrack is the linux distro geared towards security, it has its roots in two different linux distros. Backtrack version 1-3 were livecd's with some capacity to be installed.

Backtrack 4 is the first version geared towards permanent installation whilst still having its very useful livecd functionality.

Offensive Security is the official course material designed to train you in the use of backtrack. Of course there is extra stuff in there some of it not related to backtrack (ie. Core Impact) but that is because Offensive security is not just about backtrack.

IMO how much you get out of OS101 depends on your background and how tech savvy you are. If you know quite a bit about networking, programming etc, it will be dead easy. If you are lacking both, it will be challenging.

Before I took OS101 I had a background working with windows, although I am trained as a network engineer and I have RHCT training for server administration. Therefore I found it quite simple to follow. I lacked programming expertise and those modules were the hardest for me to understand.