"An area of a coin struck by a die that has a complete break across part of its surface. A cud may be either a retained cud, where the faulty piece of the die is still in place, or a full cud, where the piece of the die has fallen away. Retained cuds usually have dentil detail if on the edge, while full cuds do not."
Cited from another online coin related forum glossary section.
As far as value, it's hard to say. The saying "beauty is in the eye of the beholder" applies since some people find these highly collectible, while others wouldnt pay more than face value. I can ask one of our local dealers who specializes in error type coins what fair market value is if you would like.
My hobby: collecting real money 1 copper cent or nickel at a time.
I found a nickel with a cud almost as big as that coming from the bottom. I was told by a dealer that it was worth about $0.50. Mine wasnt in great condition and that has an affect on the price.