Beth, BMI and BF are different measurements, you're right. BF% represents the percentage of your body that is not lean mass - that is just fat, only fat, nothing but FAT. Body Mass Index, or BMI, is a measure of weight that takes height into account. While it's possible to have the same BMI and the same BF% it's not necessary. The relation between fatness and BMI is influenced by age and gender. For example, women are more likely to have a higher percent of body fat than men for the same BMI. At the same BMI, older people have more body fat than younger adults.

It's easier if you use and example:

If we take Trainerdans stats and enter them into the BMI calculator we get a BMI of 27.5 - Dan's bodyfat is close to being in the single digits (10%). By BMI standards he's overweight, by body fat % standards he's in amazing shape. Now we've all seen his photos - which measurement do you think is correct here ?

As for measuring BF% online, there are a number of calculators you can use. You should keep in mind that none are 100% accurate. They are useful in monitoring progress over time, however. Pick one and stay with it.

You can find a few of the calculators in this thread

Nat