It's a diet based on one's blood group, where,
for example one group would eat lots of protein, another would eat lots of veg and another would eat carbs.
The idea is ancestral - e.g. Blood type 'O' were hunters and so they are more 'adapted' to digesting these types of foods (meat, red peppers). I tried it and found it a bit dull - you get foods you are allowed loads of, some you are supposed to limit and some which are forbidden, did not really suit me but I know 2 people who had great success on it and feel really good on it too. There is a book and cookbook about it I think.