Just like any other business the search engines have competition. They are competing for me and you. What do we want? We want the fastest and most relevant results. The search engine that is perceived as having the most relevant results and delivering them the fastest will be our choice.
As a result, search engines invest a tremendous amount of time, energy and capital in improving their results. This includes performing extensive studies of user responses to search results and comparing their results against those of other search engines.
Search engines generate revenue through paid advertising. The largest paid advertising is the pay per click model. This model is great for advertisers because they only pay if a website visitor clicks on their ad. Because the success of the search engine depends so greatly on the relevance of the search results, manipulation of the search engine results for non-relevant keywords are dealt with very seriously.
Each major search engine has a team of people who focus solely on finding and limiting spam from their search engine results. This means the SEO working on your site needs to be careful of the tactics he or she employee. If the search engines see what you are doing as spamming it could carry penalties.