B bulbs: B bulbs are more like the sun and will tan the top layer of skin. You will tan in these beds in the same way you tan in the sun (i.e. if your skin is sensitive to the sun you should limit your exposure until you have built up a base). Good for people who tan easily or like the color they get from the natural sun. Great bulbs for those taking a vacation.
A bulbs: More of a bronzing bulb. It tans from the third layer of skin out, which is why it will take a couple of visits to show true results, but it will last longer. Provides more of a bronze look and should not burn. Very good for fair people.