The B refers to the shape of the bulb. Then there is a number that follows that your lamp is probably a b-10. The number refers to the largest diameter of the lamp in eigths of an inch. Lamps can also be identified by their base and type eg mh400/m/u which means metal halide 400 watts mogul base universal burning position
for more detailed information see the following link
http://www.adventurelighting.com/guide_intro.htm
