It's pretty much a glorified tinted moisturizer. It's a bit more pigmented (by that I mean it provides more coverage) than the regular tinted moisturizer but usually not as much as a foundation. I tend to like BB creams that are on the thicker, more pigmented side, but that's just me. Oh, and it stands for "beauty balm" or "blemish balm".
Some companies like to make it seem like their BB cream can do 84932 things when in reality all it's going to do is even out skin tone (but usually not as much as a regular foundation but more than a tinted moisturizer) and hydrate the skin (more so than foundation and I'd say almost as much as a tinted moisturizer).
I use a BB cream instead of foundation for my everyday routine. I love them. And for me, the thicker, the better. My favorite, currently, is by Missha (the one on the left). It's a Korean brand. A lot of people say they feel some of their colors are too ashy, but I have pink undertones that I don't like so it helps even them out. I'm also a fan of Maybelline's BB cream. It's highly pigmented (at least the medium color is) and I think it evens out skin tone very well.
If any of you kittens want to share your thoughts on BB creams, leave a comment :) Meow