Who are the largest online advertisers?

Pay per click advertising or PPC advertising is an arrangement in which webmasters (operators of web sites), acting as publishers, display clickable links from advertisers, in exchange for a charge per click. As this industry evolved, a number of advertising networks developed which acted as middlemen between these two groups (publishers and advertisers). Each time a (believed to be) valid web user clicks on an ad, the advertiser pays the advertising network, who in turn pays the publisher a share of this money. This revenue sharing system is seen as an incentive for click fraud.