Currently available fat replacers can be grouped into three basic types
First are those that thicken and moisturize food and have some of the "mouth-feel" of fat but that are actually carbohydrate or protein and are caloric. These are called fat "mimetics".
Second are compounds that have the same chemical properties as fats (they don't mix with water for example) and are non-caloric (or barely caloric). These are called "lipid analogues".
Third are "emulsifiers"; essentially real fat that either has a somewhat lower calorie density than ordinary fat and/or that can be used in lower amounts to achieve the same effects (mouthfeel, moisturization) as larger amounts of ordinary fat.