Airstones are the porous fittings that go on the end of an air hose to make the fizzy-bubble effect in your aquarium.  They come in different shapes and sizes so you can choose one to suit how you want it to look.  Little round ones are great for tucking inside an ornament or behind a rock, and the larger ones look better laid behind features to give a bubble-curtain effect. They are all compatible with the standard air hose.