Also known as "Google juice"
One of the greatest features of the Google search engine has been that it does not simply trust a web page to give an accurate description of its contents. Other search engines, such as Alta Vista and Lycos have had their results skewed by trusting the keywords and content of the web page, and some authors have loaded those with extra words to attract higher results. Instead, Google has given plenty of importance to how other sites link to a site - if lots of sites point to Everything2 for example, calling it an "user-contributed encyclopedia", then if someone searched for "user contributed encyclopedia", Everything2 would show up high in the rankings.
However, as of late February and early March of 2002, some people have found a way to make use of this fact. If enough people link to a site with the same or similar phrasing, they can get the linked site to the top, no matter what it's content. The weblog community on the internet has discovered this, and started making use of it. To influence Google's ratings in this manner has become known as a "Google Bomb".
The first known Google Bomb was created by Adam Mathes, as a joke. He had discovered, while searching for "internet rockstar", that Ben Brown showed up as number one, though he never used that term on his page - other people had linked to his site in that manner. With the collaboration of a number of other weblogs, he managed to get the phrase "talentless hack", when entered into Google, to point to his friend's web site.
Since then, it has slowly picked up, with groups of weblogs working in Tandem, and as a result, affecting results. For example, one person received a telemarketing call from Critical IP, a company who was cold-calling the whois database for a list of people to sell things to. One weblog author put a complaint up on his weblog, and soon others were pointing to it. Before long, a websearch on Google for "Critical IP" brought the complaint up as the highest result, above the company's own website. The Church of Scientology has been using this method also, managing to get a number of pro-Scientology sites up and pointing at each other, in the hopes of lowering the ranking of Operation Clambake, the anti-Scientology site. Apparently at one point, people even managed to get a search for "dumb motherfucker" to point to the web site of George W. Bush.
The key is that Google will index frequently-updated sites more often, originally intending this to help them keep up to date on news sites, like CNN or MSNBC. However, many weblogs are also updated frequently, and thus they have a larger effect on Google's results.
Many have pointed out that the effort and coordination it takes to do this shows that Google's algorithms work, and work well, making influencing the results difficult, but not impossible.