The primary differences are that some fat-free fryer models require a small amount of cooking fat to operate and may not use a halogen heating element. Fat-free fryers don't use a single, standard design, and some operate in a manner that is very similar to that of halogen ovens.
Both halogen ovens and fat-free fryers are types of convection ovens, in which one or more fans circulate heat from a heating element to cook food evenly. In halogen ovens, the heating element is always a halogen lamp. A fat-free fryer may use an electric heating element or some other alternate type. Some types of fat-free fryers also require a spoonful of cooking fat, which is not required for halogen ovens.