A BB cream can be worn alone for light coverage or before applying foundation or mixed in with foundation. The purpose of BB cream is to reduce redness, appearance of scars and even out the skin surface. Basically the BB cream is supposed to take the place of a primer, moisturizer, tinted moisturizer.
Bb creams can be used to replace your moisturizer and foundation. They are a combination product of treatment, moisturizer, sunscreen and makeup. I prefer to still use a moisturizer because I have drier skin, but that's just me. Also, many people like to "set" their bb cream with powder, just like you would do with a foundation. Some bb creams (like Urban Decay's) is meant to be more of a foundation primer. Read the individual descriptions on Sephora's website for exact application info.
I love bb creams to layer under foundation. They act like a primer, have SPF, etc. Some even ad luminosity. I also often wear it alone when I'm just going out to run errands. Think of it as a charged up tinted moisturizer. My favorite is smash box. It has decent coverage and more color options than the other brands. Always put regular moisturizer on first and let it sink in first. Then apply be cream with your fingers. Rub it in like lotion, not like foundation.
I've only just started to use this, but I put it on in place of my usual liquid foundation, or instead of foundation and powder. It's kind of a high class tinted moisturizer, but works better than any tinted moisturizer I've ever tried.
How you use bb cream would depend on what you want from your beauty routine. BB cream will moisturize, smooth, and prime your skin. It also has a slight tint to even out your skin tone. I have dry skin so I layer it over my moisturizer, and I have fairly even skin with no breakouts so I find that the light coverage the BB cream provides is all I need.
The BB cream would be perfect for a day at the beach, since it provides enough coverage for a confidence boost, and it is also waterproof and has sunscreen (although you would have to re-apply every so often just like sunscreen)
While the BB cream alone looks great, it can also be a great base for your foundation if you prefer more coverage. When I have a formal occasion or decide I need more coverage, I layer a powder foundation over my BB cream. I find that the light coverage powder foundation plus the light coverage BB cream combine well to hide all of my imperfections while looking completely natural.
bb creams or beauty balms provide skin benefits along with light coverage. Depending on which bb cream you choose you will get different skin benefits (like moisturizer, spf, primer, etc) and different levels of coverage (most give only light coverage but can be layered for more coverage). You apply it after washing your face (on dry skin) as its suppose to moisturize, prime, cover (light coverage, not as much as foundation), and provide SPF amoing other benefits. I still use moisturizer and makeup primer before applying bb creams (or tinted moisturizer) because I feel it gives a bettter effect. But you could just apply this after washing/drying your face followed by powder if you need it (to add coverage and/or prevent oiliness) and eye/lip/cheek makeup. This takes the place of foundation as it gives some coverage, but does not give full coverage like a foundation so if you need full coverage using a bb cream alone won't do (you can use concealer and powder on top of bb cream for more coverage).