Also written bra-ket. The product of a bra, <b|, with a ket, |a>, is written <b|a>. If you're working with vectors, it's the dot product of the bra and the ket. If you have more esoteric things, like wavefunctions, you just multiply them.
Brackets are useful for finding the probability that a measurement will yield a given result; the probability that a system in a given state X will be measured by an operator with eigenstate B is |<X|B>|2