The answer to your question is "both". I think of BB's as tinted moisturizers smarter big sister. There are BB's in the color world that in my opinion offer more coverage (Smashbox, Too Faced, Dior), then there are BB's in the Skin Care World that have lighter coverages, (Dr. Jart, Boscia). So basically you have options. I like the way my BB's look and feel on their own. But since they have a primer quality to them, you could wear them like a primer under your fuller coverage foundations. Also, BB creams have SPF in them, so again, you have options. Let's say you love your foundaiton, but it doesn't have an SPF - you can use your BB as your moisturizer (if you have oiler skin), as your primer, and as your SPF under your foundation. When you don't want or need the coverage of your favorite foundation, you can wear your BB on it's own.
I hope that helps!
B.B. is used in place of a tinted moisturizer, as the previous person stated. I've been told by Sephora cast members, that you can add foundation on top of the B.B. if you want more coverage. I could see doing that only if I were to want to create an evening look after work. The B.B.'s are used in place of foundation if you want to create a natural and even look. I just found out that Sephora is going to have Beauty IQ in the Sephora nearby. I'm not sure if your area has this station yet. It's supposed to let you know what your skin care concerns are and how to best address them. I'm excited to see how that works.
Frankly it is a way for the cosmetic industry to make money. I can tell you tat you don't need this. If you want you can make a tinted moisturizer yourself by mixing a very small amount of Foundation with the moisturizer!
Also-- remember that you need to stick to things that have been proven to work for many years , leave Primers and BB creams alone as they are new.
This is my best advice to you as a make up artist.