Airsoft Grenade Guide:Airsoft grenades are generally categorized into the few following types:

Hand Grenade: Where you pull the pin, and it actually release the gas / bb to simulate an explosion.
Dummy grenades: Where it is for decorative / simulation purpose. Or as a bb storage portable container.
40mm Grenade Launcher Grenades: Where it is to be filled with bb and gas for the Airsoft grenade launchers that takes 40mm airsoft gas powered grenades. (To reset it, press the button down until you hear a click, then release it. If you twist open the grenade it will void all warranty.)