There are essentially two strands of SEO – there’s the industry standard whitehat techniques and there’s blackhat techniques. Both have been used extensively in SEO circles, however, blackhat is certainly seen as the more risky and unethical way of doing things!
Whitehat SEO is a way of doing SEO which is seen to be the best, healthy and ethical way of working. This involved building up trusted links through dialogue, sharing and commenting on blogs, getting your name out there and noticed by industry experts and enthusiasts. This is a long and drawn out process involving a great deal of hard work and time, but the overall results are worth the effort, as search engines place a high emphasis on this fair way of working.
Blackhat techniques involve lots of quick shortcuts to getting your site ranked, including buying multiple backlinks (not always high relevancy, thus worthless) and paying for other types of links to and from your site in order to manipulate search engines into ranking you higher for your keywords. These are seen as poor ethics and unfair to those companies trying to do things properly.