Here is the difference between Wikipedia Encyclopedia and the newly introduced Google Knol.
Wikipedia

Wikipedia was founded in January 2001. It has more than 8.2 million articles in more than 200 languages, including more than 2 million in English.
Wikipedia is not ad-supported, but instead supported by the nonprofit Wikimedia Foundation.

Google Knol
Google Knol, is a user-generated encyclopedia, collaborative reference site.
Knol allows its authors to earn a profit from their articles with ads.

Google Knol, is a user-generated encyclopedia, collaborative reference site. It’s similar in concept to Wikipedia, except that Knol allows its authors to earn a profit from their articles with ads.

“Our goal is to encourage people who know a particular subject to write an authoritative article about it,” according to Google.

Knol makes it easy for you to write and share your knowledge with the world.

Knol offers:

• Ease of use
  All you need is an account, a name and a desire to write and we’ll take care of the rest.
• Control
  You specify the level of collaboration you want with the community. Your knol, your voice.
• Community
  You can connect with other experts in your area of interest to share and grow knowledge.
• Visibility
  We value and promote authorship. Great content will be visible on any search engine.
• Growth
  Sharing your knowledge with the world is rewarding for everyone.

Visit http://knol.google.com/ for user-generated encyclopedia.