Fake foods are made from materials that are combined to masquerade as food; however, they are not an example of what they advertise, and often should not be eaten (this last is extremely important. The materials used to make fake food are sometimes extremely poisonous and destructive). Fake food is generally used in advertising.
There are two primary situations in which it is used: When an advertiser wants to show the viewer a specious example of the culinary item he is attempting to sell (which is, of course, never as succulent and beautiful as the shown one), and when the advertiser wants his example of the item he is attempting to sell or demonstrate to last for a very long time.
In the first case, the materials that compose the fake food may actually be edible, and on occasion even be from the same stuffs as the actual item being sold. However, it might use a comparatively disporoportionate amount of meat, cheese, dressing, or whatever. The best example of this is in the case of burger commercials, in which a burger of vast size is advertised, while the actual product is in fact an emarrassingly small piece of garbage.
In the second case, imperishable items are generally used, and therefore the fake food here is almost never fit for consumption. For example, a resturant may want to advertise what their desserts look like, so they create a replica of, say, their ice cream dishes. Of course, they can't use real ice cream, since it wouldn't last any time at all. Instead, they might use a scoop of lard or something.
I consider the use of fake food in this manner to be deceitful, and corporations that do this should be thrashed.