Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings, and could be considered a subset of search engine marketing. The term SEO also refers to "search engine optimizers," an industry of consultants who carry out optimization projects on behalf of clients' sites. Some commentators, and even some SEOs, break down methods used by practitioners into categories such as "white hat SEO" (methods generally approved by search engines, such as building content and improving site quality), or "black hat SEO" (tricks such as cloaking and spamdexing. White hatters say that black hat methods are an attempt to manipulate search rankings unfairly. Black hatters counter that all SEO is an attempt to manipulate rankings, and that the particular methods one uses to rank well are irrelevant.