I think we might be getting tripped up (as far as two types of poly) on the actual word. There ARE two types of poly, polyandry and polyamory.

polyamory n :*participation in multiple and simultaneous loving or sexual relationships

polyandry n the practice or condition of having more than one husband at one time. (also called polygamy)

The first is the common one among BDSM and D/s couples, especially in O/l relationships. The second, whether formalized by an actual civil or religious ceremony or not, is much less common.

Earlier answerers have provided some fine information. I just hoped to clear up the wording.