Spamdexing (a portmanteau of spamming and indexing) refers to the practice on the World Wide Web of deliberately modifying HTML pages to increase the chance of them being placed high on search engine relevancy lists. People who do this are called search engine spammers. In layman’s terms, spamdexing is using unethical means known as “black hat seo techniques” to unfairly increase the rank of sites in search engines. When a website is optimized to be indexable by a search engine, without trying to deceive its web crawler, this is called search engine optimization. To be sure, there is much gray area between white-hat search engine optimization and black-hat spamdexing.
Blog, wiki, guestbook, and referrer spam
Google’s PageRank system uses the number of links to a page as an index of its “importance”. Ordinarily, very few pages will link to a spammer’s commercial site, because it is of no interest to anyone else, and hence it will have a very low PageRank score. To counter this effect, spammers attempt to create links to their sites on other people’s pages.
The most common targets for this kind of spam are weblogs, the spamming then being known as blog spam, or “blam” for short. In 2003, this type of spam took advantage of the open nature of comments in the blogging software Movable Type by repeatedly placing comments to various blog posts that provided nothing more than a link to the spammer’s commercial web site. 
Similar attacks are often performed against wikis and guestbooks, both of which accept user contributions; something that consistantly impresses and confounds critics of Wikipedia is its remarkable lack of spam, in spite of having nearly one million articles and over two million pages.
On January 18, 2005, Google proposed a
rel="nofollow" attribute that could be placed on a link; doing so instructs most major search engines to ignore the link, rendering it useless to spammers. Software is then rewritten to add this attribute to any link embedded in a comment. As of April 2005, nofollow has seen expanding usage, but is not yet universal. 
As well as comment forms, editable pages and guestbooks, some sites publish a list of the most common referrers to their site in order to show how readers have found it. These lists have also been exploited by spammers with so-called referer spam, in which the spammer makes repeated web site requests using a fake referer URL pointing to a spam-advertised site. That URL will later appear as a link on the site, boosting the PageRank of its target.
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