Nutrisystem is different from most diets because instead of giving you information on how to lose weight, they actually send you complete meals through the mail. If you've tried creating your own diet but got frustrated with counting calories, measuring food and restricting what you ate, this plan may work for you.
You pick the foods which are then sent straight to your door. Assuming you don't live across the street from a Burger King, cheating on this diet is a lot harder since you don't have a lot of extra food in your fridge (although you can just eat more than 1 portion).
The downside is that variety can be limited. Sometimes popular choices are sold out in which case you'll be sent an alternative that might not taste as good. When you cook your own food, you can customize it anyway you want to. With Nutrisystem you're stuck with what they give you.
Diet plans such as these can work just like any other diet. As long as you're burning more calories than you're eating, the weight will come off. All Nutrisystem does is make it easier for you to eat less.