A googol is 10 to the 100th power (which is 1 followed by 100 zeros). A googol is larger than the number of elementary particles in the universe, which amount to only 10 to the 80th power.
The term was invented by Milton Sirotta, the 9-year nephew of mathematician Edward Kasner, who had asked his nephew what he thought such a large number should be called. Such a number, Milton apparently replied after a short thought, could only be called something as silly as a "googol."
Later, another mathematician devised the term googolplex for 10 to the power of googol - that is, 1 followed by 10 to the power of 100 zeros. Frank Pilhofer has determined that, given Moore's Law (which is that computer processor power doubles about every 1 to 2 years), it would make no sense to try to print out a googolplex for another 524 years - since all earlier attempts to print a googolplex out would be overtaken by the faster processor.
Larry Page and Sergey Brin, the founders of Google, named their search engine after the term googol. In 1997, Larry was brainstorming names with other Stanford graduate students, including Sean Anderson, and looking at available domain names. Anderson miskeyed googol as "google" and found it available. Larry liked it and the name "Google" stuck. Google's corporate headquarters is called the GooglePlex, an affectionately tongue-in-cheek reference to the origins of the company name.