Leaf or sheet gelatin is the same substance as granulated gelatin, just packaged and sold in a different form. It is much more widely used in Europe (and shows up in more European recipes) than in the States.

The gel-making ability of sheet gelatin is constant no matter what its size, so four leaves equal the amount of gelatin in the standard 1/4-ounce packet sold here (or one teaspoon of gelatin sold in bulk). Leaf gelatin dissolves a little less readily than granulated gelatin, which is surely another reason that it is not as popular in our speed-obsessed kitchens.