They're all big contenders when it comes to the world of cake decorating. But what exactly are each of these three confectionery concoctions? Here's a brief primer:
Fondant: Generally when it comes to cake decorating, rolled fondant is the type in question. Rolled fondant is a pliable, dough-like icing which is popular for use on occasion cakes. It is made of sugar, water, gelatin and food-grade glycerine. Its smooth appearance gives cakes a polished look, and rolled fondant is also flexible and workable enough to mold into shapes, which is very effective for decorating cakes.
Gum Paste: Like fondant, gum paste is a pliable dough which is often used for cake decorating. However, instead of gelatin it is made using egg whites, confectioners' sugar, and shortening. It can be rolled quite thin and is ideal for creating hand-modeled flowers or other intricate decorations. While fondant will remain soft, gum paste dries quite hard and is better suited for decoration on a cake than for, say, covering an entire cake.
Modeling Chocolate: This is a chocolate paste made by melting chocolate and combining it with corn syrup or simple syrup and then kneaded until it reaches a stiff, pliable consistency. Used like clay, this modeling chocolate can be molded into a variety of shapes that are not as easily performed with the softer fondant. Modeling chocolate can be made from white, dark, semi-sweet, or milk chocolate. White chocolate is the easiest type to tint in colors.
Of course, while this is helpful, it's always nice to have a visual, so here are a few more defining differences between fondant, gum paste and modeling chocolate. Click to Enlarge.