Lately, group-buying sites and other companies that use discount-deals as the core of their business have become a red-hot trend, with Groupon spreading through cities around the world at a frantic pace and countless competitors and clones trying to catch some of the spotlight. Perhaps the reason for the sites’ traction is that the group-buying model is easy to understand. For those that haven't tried them, companies like Groupon find local restaurants, spas, or other businesses that are willing to provide large discounts, provided that their name is spread to a number of new customers. Groupon advertises the business by offering the coupons online, and takes a cut of the money spent on them. However, there are plenty of variations on the model and the differences can be confusing, so we’re going to give a primer on some of them below (note that there are plenty of similar companies abroad — these are some of the big names in the US, though some of the companies below are international too).