A diet plan is a plan of what to eat, when to eat it and how much you should eat. Diet plans are important because they help the body maintain a healthy weight. Diets can be categorized into different types, such as low-calorie, high-calorie, and healthy diets . There are a lot of different diet plans that claim to work for you. Most of them are not written by doctors, but rather by fitness experts and personal trainers. This is because they want to sell you their products and services, which can be quite lucrative.