Wikipedia, the popular online encyclopedia written and edited by the masses, may have a new challenger in Google Inc., which is testing a service that enlists users to write about the breadth of human knowledge.

The product, Knol, short for unit of knowledge, is open by private invitation only and is not yet public.

Users will be able to edit pages about individual topics, from history to entertainment to how-to guides. But the idea differs from Wikipedia on several points.

Whereas Wikipedia allows anyone to add information to an article, Google will limit access to a single person. Furthermore, Google is defying Wikipedia's anti-commercial stance by allowing Knol authors to accept ads on articles and to share in some revenue - much like Squidoo, an amateur encyclopedia-like site.

Google's ambitions appear to be designed to steal the thunder from Wikipedia, the niche leader, whose pages are often the top results in Google's search engine. In premiering a competing service, Google could profit by getting its pages near the top of the results list, although the company insists that it won't give Knol pages an advantage in its search algorithm.