Which One Is It? 
Whatever you choose to call it, its a necessity if you care about your physical appearance and general well being. A diet/eating programme is not only relevant for helping someone to lose weight, its important for maintaining a healthy weight and healthy body, the main differences in most eating programmes is the amount of calories and the types of foods.

This can vary massivley dependent on individual programme goals Following a programme which is too low in calories will result in quick weight loss which is not good as you would generally only lose water which you will re-gain quickly.