What is natural born citizen?

A post from RedCounty.com caught my eye. There is a federal lawsuite filed by Philip J. Berg, Esq. complaint for Obama is not qualify for office of President of United States of America. So far, I believe the natural born citizen is the citizen born on the U.S soil, U.S ship, U.S airplane. The more I read, the more I confuse. So, Obama is the U.S. natural born citizen or Obama Kenyan natural born citizen? How many citizenship Obama have? Kenyan, Indonesian, American?

Here are copy of lawsuit and other related information:

• Berg vs. Obama, Civil Action No. 08-cv-4083
• More Evidence Forthcoming On Obama's Birth Certificate - Change You Can Believe In?
• ObamaCrimes.com
• Clinton, Obama Agree: McCain’s a “Natural Born Citizen”
• Is Obama a Natural Born Citizen?
• Blogger manipulates birth certificate image, undermining Obama claims
• Obama Sued in Philadelphia Federal Court on Grounds he is Constitutionally Ineligible for the Presidency