Black Hat SEO

Stealing content - This is one of the commonest, and most despicable practices. A large site, or one that is well set up, can simply copy content en-masse from a smaller site. Since the larger site will dominate Web listings, the smaller site's page can be buried in "duplicate pages" that are not listed by search engines. Duplicate content is often suppressed by the search engines in the results. However, search engine criteria for decided which page is the "original" and which is the duplicate are defective. Therefore, your entire 30 page Web site can be simply copied by someone with a much larger site, and all your content will more or less disappear in search engines. The irony is that people who copy entire articles from your Web site to their site or Web log often think they are doing you a favor by "popularizing your work."

Keyword stuffing - Filling a page with junk content (also "spamming") that repeats a keyword in every sentence: "Widgets are good. This page is about Widgets. Paris Hilton loves Widgets."

Bait and Switch - This is usually known as cloaking. The Web site or page is set up to look like it is about some popular keyword, but really it is about something else entirely. In general, the content seen by the visitors on a page should match the content that the spider sees. No matter how you achieve the difference between what the spider sees and what the visitor sees, it is probably Black Hat SEO, though you might be able to get away with some techniques more easily than with others, since the search engines don't have an automated solution for finding the deception. If your competitor find it and complains, you might be banned.

Invisible Text - Repeating keywords in white text on white background.

Keywords in Comments - Putting keywords in HTML comments in the hope that Search Engines will "think" it is part of the text is sometimes claimed to be a Black Hat practice that can lose ranking for your page.

Doorway pages - Pages that are full of the relevant content, but are meant to be seen only by search engines. A user who click on that page result from a search engine is redirect to another page, or the page is hidden using a technique called "cloaking." Doorway pages should not be confused (though they often are) with legitimate portals or main pages that head sections in Web sites or that may serve as landing pages (pages that are the target of paid advertising campaigns.

Cookie Stuffing - Redirecting visitors, without their knowledge, to other pages. This is used mostly to get visitors to pages where they will supposedly buy merchandise through "affiliate links." An affiliate link is a special sort of advertising link that gives the Web site owner a commission on each sale.

Link Farms - A link farm is a Web site created for the purpose of giving other Web sites links. Unlike legitimate directories, there is usually no description provided for the sites and no objective editing. An exception to the link farm ban are special pages created for the links in the Netherlands eigenstart.nl Web site.

Paid Links - Google frowns on paid links, but practitioners insist it is a legitimate way to promote Web sites. If you are caught though, it seems Google only discounts the paid links as counting toward raising the importance of the target page, so there is not much to lose here.

Deep submissions - Some Web search engines claim that they consider it "spamming" if you submit more than one Web page from a site. This is called "Deep Submission." I am not aware that I have never suffered a penalty from deep submissions within reason (that is, submitting important pages or portals.