Groupon offers local discounts if enough people 'buy in'

If you've never heard of Groupon.com you're about to. The local coupon network is expanding quickly and now offers discounts in 45 cities, from Albuquerque to Washington, D.C.

The way it works is that Groupon negotiates a steep discount -- generally about 50% off -- with a local merchant. But to get the deal, a certain number of people have to "buy in."

Groupon = group +coupon.