Search engine companies (like Google, Yahoo!, MSN, etc.) have some pretty high standards to follow and for good reason: they need to ensure that their users are successful in finding what they were looking for.
If a search engine optimization (SEO) company tries to trick the search engine in order to get a high ranking for a website, it may work for a while but could result in a site being banned either temporarily or permanently once the search engines discover the deception. This practice is considered black hat, so they’re the “bad guys” in the SEO world. However, if an SEO vendor uses techniques that conform to the search engines' guidelines and involves no deception, they are considered white hat SEO and tend to produce results that last a long time. These companies would be considered the “good guys”.
So when you’re searching for a company to improve your website ranking, make sure you get the long lasting results of the “good guys” who practice white hat SEO.