Wikipedia’s definition of “Black Hat Optimization” is Content Spam. These techniques involve altering the logical view that a search engine has over the page’s contents. “White Hat” Optimization? – Ethical, common sense optimization practices.
This involves the calculated placement of keywords within a page to raise the keyword count, variety, and density of the page. This is useful to make a page appear to be relevant for a web crawler in a way that makes it more likely to be found.
Hidden or invisible unrelated text
Disguising keywords and phrases by making them the same color as the background, using a tiny font size, or hiding them within HTML code such as “no frame” sections, alt attributes, zero-width/height divs, and “no script” sections. However, hidden text is not always spamdexing: it can also be used to enhance accessibility. People screening websites for a search-engine company might temporarily or permanently block an entire website for having invisible text on some web pages.
Repeating keywords in the metatags, and using meta keywords that are unrelated to the site’s content. This tactic has been ineffective since 2005.
“Gateway” or doorway pages
Creating low-quality web pages that contain very little content but are instead stuffed with very similar keywords and phrases. They are designed to rank highly within the search results, but serve no purpose to visitors looking for information. A doorway page will generally have “click here to enter” on the page.
Link spam takes advantage of link-based ranking algorithms, such as Google’s PageRank algorithm, which gives a higher ranking to a website the more other highly ranked websites link to it. These techniques also aim at influencing other link-based ranking techniques such as the HITS algorithm.
Involves creating tightly-knit communities of pages referencing each other.
Putting links where visitors will not see them in order to increase link popularity. Highlighted link text can help rank a web page higher for matching that phrase.
Spam blogs, also known as splogs, are fake blogs created solely for spamming. They are similar in nature to link farms.
Mirror web sites
Hosting of multiple websites all with conceptually similar content but using different URLs. Some search engines give a higher rank to results where the keyword searched for appears in the URL.
Taking the user to another page without his or her intervention, e.g. using META refresh tags, Java, Java Script or Server side redirects.
Cloaking refers to any of several means to serve a page to the search-engine spider that is different from that seen by human users. It can be an attempt to mislead search engines regarding the content on a particular website. Cloaking, however, can also be used to ethically increase accessibility of a site to users with disabilities or provide human users with content that search engines aren’t able to process or parse. It is also used to deliver content based on a user’s location; Google itself uses IP delivery, a form of cloaking, to deliver results. Another form of cloaking is code swapping, i.e., optimizing a page for top ranking and then swapping another page in its place once a top ranking is achieved.