A Chinese delicacy, century eggs are duck eggs that have been preserved in a saline solution of ash, lime, salt, clay and rice for weeks or even months. When preservation is complete, the shell of the egg will become speckled, giving it an aged appearance. The yolk will become dark green, creamy and cheese-like, and the whites will turn into a dark-colored jelly.

Century eggs have an extremely strong flavor and odor that resemble sulfur or ammonia. The yolk absorbs most of the fermenting solution so the concentration of flavor is located there. Century eggs can be eaten by themselves or served with pickled ginger, but are more likely to be cooked in Chinese rice porridge (congee). You can find century eggs in Asian grocery stores or at Chinese dim sum restaurants serving congee for breakfast.

Century eggs are also called millennium eggs or thousand year old eggs.

References:

http://www.wisegeek.com/what-is-a-century-egg.htm

http://www.huffingtonpost.com/2013/02/13/century-eggs_n_2672216.html