BC is all of AB plus more.
If you are taking the AP exam for college credit, passing the Calculus AB exam will get you credit for Calculus 1 in college.
If you can get a good score on the Calculus BC exam (usually depends on the college, if they want a 4 or 5), you may be able to get credit for both Calculus 1 and Calculus 2. If you don't get a high enough score (like a 3, you may still get credit for Calculus 1. It depends on what your college accepts, though.
At least, that's what I was told.