It changes daily (and isn't always a restaurant).
With Groupon (and other similar sites), you get an email everyday telling you the Groupon of the day - which you can then decide to skip or buy. I could be a discounted gift card to a restaurant, something at a spa, gym membership/classes....
If you Google Groupon in your city, the site allows you to look at recent offers (you can only buy the one for that particular day), so you can see what the selection is like.