Ask HN: Can someone explain how Groupon works?

I'm curious how the model works. Also, do you think they seek out partnerships with a lot of shops, or only franchise businesses who can help them scale. How does the Groupon team scale? Are they mostly business dev. and sales guys? or they build something nice and wait for the shop owner to discover them, offer coupons etc. How do the businesses afford to give 90% off on items and still profit? Basically I want to get an understanding of Groupon as a customer, but also as a curious startup guy. Hopefully some of you guys can shed light on the business side of things as well. Thanks