It's a service that puts owners together with renters; i.e. homes, apartments, lofts down to simply a room in someone's house, with someone who wants to stay somewhere. You can filter out how much you're willing to pay, and the owner can choose whether they want to rent to you or not based on reputation on the site, past rentals from others, etc.
Wife and I are going to San Fran in October and I just came to realize how ridiculously expensive hotels are there. $400+/night for a bed and pillow at the Hilton, and that comes with nothing else, wtf. Vegas has already spoiled me on hotels, but that just seems extreme. I friend told me about the site and I found a couple potential options, but I don't know anything about it, whether someone could just choose to not rent afterall at the last minute, if there are any safety concerns, etc.
Married in Vegas - June 2011