It's a web site that markets for the casino. Usually the site has "affiliate links" that will take you to the casino by clicking on a banner ad; if you then register, deposit, and play, the web site operator will get a % of your losses.
Usually 35--50%
Most of the "objective review sites" and "concerned webmasters providing information" are getting paid in this fashion, instead of the model of the casino paying up front for advertising space (as is done here and on some of the better sites)
I don't like affiliate programs because the webmaster has a lot of incentive to deceive players to deposit in a bad casino (Cirrus, anyone?)and very little incentive to recommend good casinos that do not have affiliate programs. We also see webmasters coming to message boards and posing as players to defend bad casinos.
Webmasters can also be cheated in this system, because they make the initial investment and effort of advertising, and the casino keeps the books. Casinos have been known to short pay webmasters, claiming fewer players came through the links, or that the players own and did not lose.
A new twist on affiliate marketing is the "template site" or "private label casino". It's a url for downloading BetheDealer or Video Poker Palace software, with a casino name on the felts etc. You think it's a different casino, but it's really an affiliate site; the "owner" has nothing to do with running or operating the casino. Promotions, service, accounting, etc. are all from one central site. This can make it difficult to get problems resoolved.