SEO can be split into two distinct camps: white hat SEO and black hat SEO (with, of course, some grey hat-wearers in-between).
Black hat SEO refers to trying to game the search engines. These SEOs use dubious means to achieve high rankings, and their websites are occasionally black-listed by the search engines.
White hat SEO, on the other hand, refers to working within the parameters set by search engines to optimise a website for better user experience. Search engines want to send users to the website which is most suited to their needs, so white hat SEO should ensure that users can fid what they are looking for.