Keyword advertising is a form of online advertising in which an advertiser pays to have an advertisement appear in the results listing when a person uses a particular phrase to search the Web, typically by employing a search engine. The particular phrase is composed of one or more key terms that are linked to one or more advertisements. The most common form or keyword advertising, focused on payment methods, is pay per click (PPC), with other forms being cost per action (CPA) or cost per mille (CPM).