Common knowledge is information that is widely accepted, such as the moon revolves around the Earth or there are 50 states in the USA. Sometimes, however, the case isn't so clear. If the information can be found through various sources that are widely available then it can be accepted as common knowledge.
For example, it is common knowledge that Antioch alumnus, Jose Ramos-Horta, is the winner of the Nobel Peace Prize, because that information is known by a large number of people. The knowledge is not exclusive to a particular domain of expertise, and it can be found in thousands of resources that are readily accessible.
"EastTimor.JoseRamosHorta.01" by Elza Fiúza/ABr - Agência Brasil [1]. Licensed under CC BY 3.0 br via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:EastTimor.JoseRamosHorta.01.jpg#/media/File:EastTimor.JoseRamosHorta.01.jpg
It is also common knowledge that Horace Mann was the first President of Antioch College. It is not common knowledge, however, that he became an educational reformer to cope with his depression after the death of his first wife.*