Basically, a 'banger' is similar to a shotgun shell, but has no shot, only a small plastic cap. You place it on a 'pen' launcher, aim it up directly over your head, and pull the trigger. It fires into the air making a loud 'bang', like a shotgun blast, which scares the bear away.
The downside is, in a panic, if someone aims it towards the bear and fires, and it overshoots the bear, you have a scared, angry bear running away from the sound TOWARDS you!
Here is a link to a "Bear Banger" from Mountain Equipment Company in Calgary.
What if you would just rather blow its head off with your Mossberg 500? :-)