Priming is the process of adding sugar to fermented beer in preperation for bottling. This process causes leftover yeast in the bottled beer to begin fermenting. This fermentation is created for the purpose of producing carbon dioxide gas which displaces oxygen gas and more importantly is over the course of several days absorbed by the beer resulting in carbonation. This is called bottle-conditioning, this natural carbonation method is supposed to taste better than the forced carbonation method. Corn sugar is often used since it doesn't add any flavors to the beer. Dry malt extract is also used.
See Homebrewing.