Do diet foods make you fatter?

I have actually thought about this for a while. Why is it called diet foods, and why do people buy diet foods? I have always thought that diet foods was food that would be very low fat and would help you lose weight, but the definition is according to Wikipedia:

Diet food (or dietetic food) refers to any food or drink whose recipe has been altered in some way to make it part of a body modification diet. Although the usual intention is weight loss and change in body type, sometimes the intention is to aid in gaining weight or muscle as in bodybuilding supplements.

According to this, diet food can actually help you gain weight? I am not sure that this definition is what people learn from the various marketing campaigns, at least I haven´t learned anything about this. And the low calorie stuff, did you know that they may actively trigger overeating? As if it wasn´t hard enough to shop quality food and drinks already.