Airsoft grenade launchers are an unusual category of airsoft gun in that it can shoot different types of projectiles that serve a purpose in milsim (military simulation) style games. The typical airsoft "grenade launcher" is a gun that requires an airsoft grenade shell that is gas powered. The launcher activates the shell which is the component that actually shoots. The type of shell you use will determine how it shoots and what it shoots.
The most common type of 40mm airsoft shell is the BB shower shell. These are gas powered shells that feature multiple "barrels" which you load to the top with BBs. These can hold as few as 50 shells to several hundred. When fired, a stream of BBs shoot out much like a shotgun blast. Other types feature a big bore which can be used to launch paintballs or foam rockets (and Nerf balls/rockets). These are great fun though you'll have to check with your local field or event holder if these are field legal. The gas grenades when fired emit a loud pop which can be startling to some so use caution.
These shells are powered by green gas and usually take a 3-5 second charge time to fill, maybe longer. As with any gas powered airsoft item, these shells will require care and maintenance to keep them in good shape and not leaky over time. The simplest thing you can do is put silicone oil on the valve and then charge the valve with gas to fill it. The gas will push the silicone into the shell and the pressure will distribute the oil to everywhere inside including the o-rings, which is the component you are trying to oil. Any small leaks can be addressed with the silicone oil and any big leaks can be detected this way. These shells have small ball bearings and complex parts. DO NOT disassemble unless you are qualified. Parts will get lost and you will void your warranty.
Q: How far do these shoot?
A: Range will depend on the type of shell used, not the launcher. Remember it's the shell doing the shooting. The shell is not the projectile but the host for the projectiles. For range expect the BB shower type to go 50 feet or less in a large shotgun blast type cloud. It's not accurate but scares a lot of people! For the ones that can launch rockets, with the right foam rocket or projectile you can see these things go 150 feet or more (arcing shots of course).
Q: Are the shells reusable?
A: Yes, you just reset the valve, fill with gas, and load up the shell again. The release valve is the part that gets activated by being pushed by your launcher. It sometimes locks into the "fired" position and will have to be reset before you fill it with gas again. You can reset it by pressing it with your finger or something blunt until you hear a click, and then have the valve return to the centered position. It takes a little practice but is easy to do.