One great way to improve your search engine rankings is to get lots of backlinks. These are links from other websites, preferably in your industry. This has led many sites to buy links on “link farms” or pay SEO companies to set up dozens or hundreds of low quality links to their site. However, the search engines, notably google, are starting to strike back.
Google recently implemented some search algorithm changes that caused a lot of sites to drop off significantly in the rankings. Here’s a article that explains some of the changes, and shows you how to recognize a good link from a bad one: