A Google bomb occurs when a group of people conspire to artificially elevate a web site in Google’s web search results ranking by linking a particular word or phrase to the website.
Google moved to curb Google bombs by tweaking their formula for ranking pages by relevancy. The changes limited the ability of relatively small groups to create Google bombs, but it did not end it entirely.
“Google bombs” are collective efforts to link to a site by a key phrase and artificially elevate a Web site in the Google search results for that search phrase. Google bombs rely heavily on the influence of PageRank. Some Google bombs are politically motivated, while others are done as pranks, and some may have been motivated by ego or self promotion more
Examples: The classic example of this is the 2003 Google bomb of the phrase miserable failure, which ranked the biography of George W Bush as the most relevant link after bloggers repeatedly used the phrase as the anchor text to link to the presidential biography.
In a bungled effort to dodge the Google bomb, the White House moved the Bush biography to a non-permanent URL that would apply to any seated president. This means that Obama or any other seated president should avoid the words “miserable” or “failure,” or the Google bomb may kick into effect again.