You load bb's into a shell, that shoots a large amount of bb's at once. They range from 18(i think) to like 160 shots depending on the grenade you get. You fill the shell up with green gass, and put the bb's in and press the trigger. I think they shoot at like 300fps. Not 100% sure. And its only worth the money if you are going to use it. Alot of CQB arenas are banning Airsoft Grenade launchers because no one wants to get hit 50 times from up close.
And yes the Classic Army grenade launchers are fine. Pretty much they are just 40mm tubes with ea trigger plunger. The grenades are what power the launchers and what you should be looking at for quality.
CA Grenades are ok for the price. I've had a few qualtiy issues with them. Madbull grenades seem to be the best right now for quality, performance, durability and cost.
Here's a little thing I wrote up about airsoft grenades a while back.
Reviews of nade launchers are all over my site too. and even a review of the Army Code, Mosquito Molds Grenade internal upgrade. In case you have any MM nades with blown o-rings that need to be ressurected. A very impressive product.