Counterpoint is a method of composing music. It is a set of "rules" that are traditionally followed in order to write music for two or more voices. It is organized into "types" of counterpoint known as species. They go in order from most restrictive to the least. The more voices there are in a musical score, the more freedom the voices have (less rules to follow). It is important to know what Consonant Intervals and Dissonant Intervals are.
Much of the credit for the formation and organization of the rules of counterpoint is given to a single man by the name of Joseph Fux. His classic and groundbreaking work on counterpoint, Gradus ad parnassum, is regarded as the most important contribution to the advancement of the theories of counterpoint. It has been historically proven that great composers such as Haydn, Mozart, as well as Beethoven worked from the same book (giving much to say concerning the usefulness and importance of this method.)