SEO (Search Engine Optimization) is common practice for most businesses with a web presence. All sites want to show up high on a search engine's index so that they are one of the first few sites listed when people search. The practice becomes problematic when black hat SEO practices are used instead of white hat SEO practices, which are preferred by search engines and many website owners alike.
Black hat SEO uses techniques considered to be less than ethical by search engines. Another name for this is "spamdexing." There are many techniques used to accomplish this. One is using link farms or dummy sites to make it look like the website is being linked to more often than it actually is. Another is using hidden text which is either invisible or colored the same as the background to appear invisible in hidden places or positioned off-screen. A third is called "cloaking." This involves giving one page if the searcher is human and a different page if it is a search engine or spider. All of these techniques work, but the risk is that many search engines will reduce the site's ranking or ban them all together if the site is caught using these techniques. This type of SEO can affect the search results customers receive, and in the case of pay-per-click advertisers, it can affect the search engine's profits.
White hat SEO is considered a more ethical and organic way of achieving higher search engine rankings. It involves taking a common sense approach to creating and maintaining a website by making it relevant to its human audience, keeping content up-to-date and easy to find, and making the site easy to navigate. Using internal links instead of drop-down menus or scripting makes it easier for both the consumer and search engine spiders to get around. Making sure the site's content is interesting and useful, not just filler, is another. A third would be reciprocal linking, in which the site links to another site which also links back. This is best done with sites that deal with similar topics or products and would therefore interest your audience. Creating good titles and meta tags, and getting the site's link out there by doing guest or forum posts or articles also work well in moving a site up the search engine ranking.
While some companies still use black hat SEO because they feel that the benefits in traffic may outweigh the risk, many companies see it as unethical and contrary to the interests both of the company and of their audience. They see using white hat SEO not only as more ethical, but as more beneficial to their company in the long run.