You’ve probably seen them before: mysterious "Links" pages or small text links at the bottom or side of a page with curious collections of words used to link to another website. You may have wondered at the time what purpose they really served, or you may not have given them another thought, after all, they’re not part of what you were looking for, so why pay them any attention? If you were a search engine spider however, the piece of software which ‘crawls’ the web on behalf of search engines, reading web pages and indexing their content, you wouldn’t realize that those links were any different from other links on the page. You would follow them, reporting back to your search engine that they were just as important as every other link on the page.
This is what some search engine spammers and ‘link exchangers’ or ‘reciprocal linkers’ rely on to promote their websites within search engines. You see, part of the ranking algorithm which decides where a website comes up in a Google (and possibly other search engines) results page relies on the links between websites to determine which sites are more important or relevant to particular keywords. To do this, links coming in to a page are analyzed and weighted based on where they come from and what text the link contained. If, for example, a website had multiple incoming links using the words "online corporate training", then that site’s ranking for those keywords would likely be increased. If the sites providing the links were ranked highly in a related field (say "business training"), then the ranking of the first site might be affected even more.
Companies now offer link-exchanging networks, where sites that are willing to provide a link on their site to another trade that service for a link back from them. In this way, both sites benefit from the value of links from other websites. There are even software programs which allow you to search for competitors and the people linking to them, to manage the process of acquiring links (via email) from other websites, and handle the creation of a page dedicated to linking to other sites from your website.
Another form of link-farming which is a little more public, and lacks the financial motives, is blatantly called "Google-bombing". A Google-bomb is when a group of websites work together to artificially place a website in Google under a specific search term. Although this technique has relatively harmless motives (more of a joke than a business strategy), it is one and the same technique as is used by commercial operators to get themselves listed in a good position against a search term which their customers might use.
Search engines will continue to combat efforts like this to affect their listings, and search engine spammers will no doubt continue to come up with new and more effective ways of getting to the top of the results. Unfortunately the open nature of the web and the sheer volume of information available means that search engines will always be subject to some form of abuse unless they allow only selected sites to be displayed. With Google indexing 4,285,199,774 pages at last count, that’s a tall order.