BMI is where they take your height and say you should weight between x and y pounds. It is an average and works for some people.
BF % is what percentage of
your weight is fat and what percentage is lean mass (This includes muscles, bones, organs and so on).
They can be very different because a body builder can have very low fat, but the BMI chart would say he is extreamly obese for his height.
For my height many charts say I can weight 100-120 pounds (Almost every chart I look at is different). However I have 106 pounds of lean weight according to my body fat percentage. That means to get down to 100 I would not only have to drop to 0% body fat which is impossible, I would have to lose muscle and bone density as well.
If you are average build, BMI can sometimes be a good chart to give you an idea of what weight to shoot for. Many overweight people have a higher lean weight from years of packing around all those extra pounds. Someone who has starved themselves and not gotten enough protien may have a very low lean weight.
The best way to evaluate your goal weight is to figure out your lean mass, then shoot for a healthy percentage of body fat. As you exercise and get adiquet amounts of protien your lean mass will increase. AS this happens you may have increase your goal weight. That can be scary for some but remember
1. A pound of muscle takes up less volume then a pound of fat, so when you reach your goal weight you may be able to wear 2 sizes smaller, and that is what people see, not a scale number.
2. Muscle has better circulation then fat. This means you burn more calories even when sitting on your tush doing nothing
3. The improved circulation also means lower accurence of cellulite.
4. Increased endurance and strength. ect ect ect