Common knowledge is information and ideas you deem broadly known by your readers and widely accepted by scholars.
Information such as the basic biography of an author, the dates of a historical event, or widely acknowledged scientific facts, do not need to be cited.
Ex. Shakespeare was born in Stratford-upon-Avon in 1564.
The French Revolution began in 1789.
The earth revolves around the sun.
Proverbs, sayings, and cliches do not need to be cited.
Ex. A bird in the hand is worth two in the bush.
Curiosity killed the cat.
As a general rule, if you have seen the information repeated in your reading you do not need to cite. But, if the information has appeared in only one or two of your sources or when the information is controversial or debated, you must cite the source.
Remember, statistics and the exact words of authors must be cited and given proper documentation.
Ex. As of 2000, the percent of people who owned their own homes in Alabama was 72.5%; while nationwide that percent equaled 66.2% (1).
Kelly Sultzbach suggests that the "characters’ sensory contact with the natural world shapes the novel’s themes of finding community and coping with the pain and loss necessary for new growth, as well as destabilizing hierarchical relationships between different classes and races of people" (2).
(1) FedStats. MapStats: Alabama. 10 July 2009. 13 January 2011 <http://www.fedstats.gov/qf/states/01000.html>.
(2) Sultzbach, Kelly. "The Chiasmic Embrace of the Natural World in Eudora Welty's Delta Wedding." Southern Literary Journal 42.1 (2009): 88-101.