BB creams resemble a tinted moisturizer with skincare benefits. They give sheer coverage, tend to come with SPF and are enhanced with various antioxidants, give primer type benefits and come in a range of shades. Some brands have only a couple of shades so matching your skintone with these ones is hit or miss. Some have skin lightening properties, especially the Asian brands.
Most foundations give more coverage than BB creams and only a few brands come with skincare benefits.