The 1200 calorie diet is a calorie restricted diet, which is about as low you can healthily go for a woman. For a man, it's about 1600 calories a day. This way, you'll take in fewer calories than you burn and you're guaranteed to lose weight over time. The danger of this diet is that when you go off of it, you will tend to overeat and yo-yo.
Make sure that you consult with your doctor before starting this diet. Sometimes a radical change in calorie intake stresses your organs adversely. Unwanted body chemistry changes result.
No particular organization "owns" this diet. There are plenty of 1200 calorie meal plans online for free.