Capers are pickled flower buds, and very salty. Look for them in the pickle section of your grocery store, or in delis. The smallest ones are more expensive, and more highly esteemed. Rinse them before using.
Caper berries are large capers.
They're sometimes used instead of green olives to garnish martinis. See the Pickled String Beans recipe posted by the Internet Chef. Drain liquid from a jar of dill pickles and replace it with a solution of two parts sugar and one part cider vinegar.