The primary way to win the SEO game is to write and present unique content through an optimized page with the keywords frequently repeated, in clear, easy to read language.
SEO expects advise editors on content to make sure that it can be easily indexed by search engine robots, while also making sure the the site is appealing to users through effectively written webcopy.
However, in recent years, some site owners were playing the SEO game in a rather underhanded manner with what are commonly referrer to as Black Hat SEO tactics, for example stuffing a page with keywords either visibly on the page, or in white text on a white background, or in the metatags, the hidden parts of the page that search engine spiders tend to look at, which well organized sites should pay special attention to (if you look at Yahoo, for example, the little mini site descriptions which appear on your search results page often come from the metatags written by the owner of the site.
Each of the major search engines use various algorithms to determine relevancy ranking. Some of these include determining whether the search term appears in the META tag, others whether the search term appears in the body text or the actual URL of a web page (using dashes versus _ between words is a much debated topic).
Many search engines check for instances of black hat SEO tactics, and will remove suspect pages from their indexes.
Search is big business due to the amount of ad revenue generated each year. Each search engine wants to deliver the best results possible to provide a fast, superior experience for their users.
So our advice is to stay strictly white hat when it comes to SEO. You might get some fast results playing fast and lose with black hat tactics, but you might also cause yourself more harm in the long run.
Build your business based on legitimate SEO tactics, and you will not see your results plummet like a stone, and you will also not have to panic if you hear that one of the search engines is changing their algorithms.