Black-Hat SEO Tactics:

Keyword Stuffing
This is probably one of the most commonly abused forms of search engine spam. Essentially this is when a webmaster or SEO places a large number of instances of the targeted keyword phrase in hopes that the search engine will read this as relevant. In order to offset the fact that this text generally reads horribly it will often be placed at the bottom of a page and in a very small font size. An additional tactic that is often associated with this practice is hidden text which is commented on below.

Hidden Text
Hidden text is text that is set at the same color as the background or very close to it. While the major search engines can easily detect text set to the same color as a background some webmasters will try to get around it by creating an image file the same color as the text and setting the image file as the background. While undetectable at this time to the search engines this is blatant spam and websites using this tactic are usually quickly reported by competitors and the site blacklisted.

Cloaking
In short, cloaking is a method of presenting different information to the search engines than a human visitor would see. There are too many methods of cloaking to possibly list here and some of them are still undetectable by the search engines. That said, which methods still work and how long they will is rarely set-in-stone and like hidden text, when one of your competitors figures out what is being done (and don't think they aren't watching you if you're holding one of the top search engine positions) they can and will report your site and it will get banned.

Doorway Pages
Doorway pages are pages added to a website solely to target a specific keyword phrase or phrases and provide little in the way of value to a visitor. Generally the content on these pages provide no information and the page is only there to promote a phrase in hopes that once a visitor lands there, that they will go to the homepage and continue on from there. Often to save time these pages are generated by software and added to a site automatically. This is a very dangerous practice. Not only are many of the methods of injecting doorway pages banned by the search engines but a quick report to the search engine of this practice and your website will simply disappear along with all the legitimate ranks you have attained with your genuine content pages.

Redirects
Redirecting, when used as a black-hat tactic, is most commonly brought in as a compliment to doorway pages. Because doorway pages generally have little or no substantial content, redirects are sometime applied to automatically move a visitor to a page with actual content such as the homepage of the site. As quickly as the search engines find ways of detecting such redirects, the spammers are uncovering ways around detection. That said, the search engines figure them out eventually and your site will be penalized. That or you'll be reported by a competitor or a disgruntled searcher.

Duplicate Sites
A throwback tactic that rarely works these days. When affiliate programs became popular many webmasters would simply create a copy of the site they were promoting, tweak it a bit, and put it online in hopes that it would outrank the site it was promoting and capture their sales. As the search engines would ideally like to see unique content across all of their results this tactic was quickly banned and the search engines have methods for detecting and removing duplicate sites from their index. If the site is changed just enough to avoid automatic detection with hidden text or the such, you can once again be reported to the search engines and be banned that way.

Interlinking
As incoming links became more important for search engine positioning the practice of building multiple websites and linking them together to build the overall link popularity of them all became a common practice. This tactic is more difficult to detect than others when done "correctly" (we cannot give the method for "correct" interlinking here as it's still undetectable at the time of this writing and we don't want to provide a means to spam engines). This tactic is difficult to detect from a user standpoint unless you end up with multiple sites in the top positions on the search engines in which case it is likely that you will be reported.

Source : http://www.beanstalk-inc.com