Search Engine Optimization techniques are classified into two broad categories: White Hat SEO and Black Hat SEO. White hats tend to produce results that last a long time, whereas black hats anticipate that their sites may eventually be banned either temporarily or permanently once the search engines discover what they are doing.

An Search Engine Optimization technique is considered white hat if it conforms to the search engines' guidelines and involves no deception. As the search engine guidelines are not written as a series of rules or commandments, this is an important distinction to note. White hat SEO is not just about following guidelines, but is about ensuring that the content a search engine indexes and subsequently ranks is the same content a user will see.

White hat advice is generally summed up as creating content for users, not for search engines, and then making that content easily accessible to the spiders, rather than attempting to trick the algorithm from its intended purpose. White hat SEO is in many ways similar to web development that promotes accessibility, although the two are not identical.

Black hat SEO attempts to improve rankings in ways that are disapproved of by the search engines, or involve deception. It tries to trick the Search Engine. One black hat technique uses text that is hidden, either as text colored similar to the background, in an invisible div, or positioned off screen. Another method gives a different page depending on whether the page is being requested by a human visitor or a search engine.

Search engines may penalize sites they discover using black hat methods. Such penalties can be applied automatically either by the search engines' algorithms, or by a manual site review.

One infamous example was the February 2006 Google removal of both BMW Germany and Ricoh Germany for use of black hat SEO. Both companies, however, quickly apologized, fixed the offending pages, and were restored to Google's list.