There are a number of black hat and harmful seo techniques that people need to be aware of so that they do not fall victims to seo companies that carry out such procedures. Google’s webmaster guidelines are constantly changing therefore an internet marketing campaign that may have worked in the past may eventually harm the reputation of a website.
Link spamming is one such search engine optimisation technique to mislead search engines in order to make them think that a particular web page is being linked to more often than it really is.
Spam farms used to be used to be used containing hundreds of websites to point to a single page. Google’s spam filters are now able to effectively exclude most spam farms from affecting their rankings.
A more advanced method is to buy a domain that has recently expired but which used to have a high page rank with lots of inbound links. In that way the new website can profit from the inbound links that may still exist. Again Google is capable to detecting when a website has changed its content significantly and should be able to filter out these instances.
Recently, Google has announced link exchanges which were once an acceptable white hat seo technique as a disallowed method of internet marketing unless the links involved contain the no follow attribute
Buying links has also been banned and google provides a useful report tool that can be used to report sites that are seen to be buying links.
Search engines also have a list of spammy and trustworthy web pages that are human generated so no matter how sophisticated an SEO technique is used, if it is not compliant with Google’s guidelines, then it is likely to get caught and penalised eventually.
A description of Google’s webmaster guidelines can be found here at http://www.google.com/support/webmasters