SEM is better for testing than SEO. Because you can immediately turn SEM paid ads off and on, it’s a great strategy for testing. You can quickly revise your ad copy, target new audiences, and change landing page content to test your new tactics. This flexibility allows you to see differences in your strategies immediately. You cannot accomplish this through SEO, as it would take too much time to make changes and monitor differences in results.

Search engines may penalize sites they discover using black or grey hat methods, either by reducing their rankings or eliminating their listings from their databases altogether. Such penalties can be applied either automatically by the search engines' algorithms, or by a manual site review. One example was the February 2006 Google removal of both BMW Germany and Ricoh Germany for use of deceptive practices.[54] Both companies, however, quickly apologized, fixed the offending pages, and were restored to Google's search engine results page.[55]

Search engine optimization is a method for sustainably influences search engine rankings. Google and other search engines calculate their search results for keywords using highly complex algorithms. The individual ranking factors and their weighting within the ranking calculation are well-guarded intellectual property that belongs to the search engines and is not publicly disclosed.

Webmasters and content providers began optimizing websites for search engines in the mid-1990s, as the first search engines were cataloging the early Web. Initially, all webmasters only needed to submit the address of a page, or URL, to the various engines which would send a "spider" to "crawl" that page, extract links to other pages from it, and return information found on the page to be indexed.[5] The process involves a search engine spider downloading a page and storing it on the search engine's own server. A second program, known as an indexer, extracts information about the page, such as the words it contains, where they are located, and any weight for specific words, as well as all links the page contains. All of this information is then placed into a scheduler for crawling at a later date.