These are commercial organisations where you can join a load of other struggling would-be slimmers and get help and advice with your diet journey. Weight Watchers, Slimming World, and Rosemary Conley are the main ones in the UK.
You can attend classes or get your advice online. Mostly these are low-fat diets. So if you love baked potatoes, sandwiches, rice and pasta this is more likely to be the diet for you. You will just have to restrain yourself with the toppings because anything buttery or creamy or cheesy is unlikely to be allowed.
It's a good option if you find the support of others in the same situation helpful, and if you need the weekly public weigh-in as encouragement to stay on the straight and narrow. If your BMI is over 30, your GP in the UK can refer you for a three-month free trial of either Weight Watchers or Slimming World.