Gelatin is a colorless, tasteless, ingredient used as a stabilizer, agent, thickener, and emulsifier in an array of foods, including gummy candy, salad dressing, marshmallows, pâte, and that most famous of jiggly foods, Jell-O. It's made from processing animal collagen (primarily cowhides and bones, as well as pigskin), and can be found in several forms.
There are instant versions, which can be added to food immediately; most, however, need to be soaked in water. Once extracted and dried, it may be rolled into sheets, also known as "leaf gelatin," or it may be ground into granules or a fine powder. At its purest, gelatin is measured by weight, and for ease of use, many chefs prefer leaf or sheet gelatin over powder to avoid the hassle of weighing powder.
Gelatin sheets often come in varying grades, which reflect the stiffness and concentration of the gelatin. Although there are many differing opinions on the (somewhat controversial) substitution of sheet gelatin for powdered gelatin, pastry chef David Lebovitz suggests using three-and-a-half sheets to equal one envelope (about 1/4 ounce) of powder. Which kind of gelatin do you prefer, and how do you cook with it?