A racket is an illegal business operation. Extortion is coercing money out of a party under threat of blackmail (which yelp is doing), violence or some other negative action done on the extorted.
To some people, racketeering is a subset of extortion. To me, extortion is the simple extraction of money by coercion. Racketeering involves the protection from both the racketeer but also from other aggressors. The Yelp model fits this nearly exactly, whereas extortion is a much broader term.
From the wikipedia article on protection racket:
>In some cases, the "protection" is little more than extortion, with no real service rendered unto the victim. Otherwise, the racketeers will warn other criminals that the client is under their protection and that they will punish anyone who harms the client. Services that the racketeers may offer may include the recovery of stolen property or punishing vandals. The racketeers may even advance the interests of the client, such as muscling out unprotected competitors.
