Basically it is called a lunchbox locker. I don't know the origin of the name other than it replaces the spider gears. I am assuming it got the name from it going into the stock carrier like a lunchbox.
Anyway's it works like this. It is always locked up until you turn then it releases so one tire can go faster than the other so you can turn.
It is popular because it doesn't take setting up the rear end to install, so anybody mechanically inclined can install it fairly easily, but it is automatic, so it takes no control from you.
The No-slip is a little more expensive and supposedly is quiet, while the other "Lunchbox" lockers click when they release for a turn, so they can be noisy.
It is kind of a high end overview of what it is, and what it does. I am not sure if you wanted a more scientific answer, or if you were just curious of what one was.