Search engine marketing, or SEM, is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages (SERPs) through the use of search engine optimization, paid placement, contextual advertising, and paid inclusion. In 2008, North American advertisers spent US$13.5 billion on search engine marketing. The largest SEM vendors are Google AdWords, Yahoo! Search Marketing and Microsoft adCenter. As of 2006, SEM was growing much faster than traditional advertising and even other channels of online marketing. Because of the complex technology, a secondary "search marketing agency" market has evolved. Many marketers have difficulty understanding the intricacies of search engine marketing and choose to rely on third party agencies to manage their search marketing.