The term cadence refers to the point at which a phrase comes to a rest. You can use this term to refer to a rhythmic or melodic cadence, but for the purposes of this class we will reserve the term for a harmonic cadence. The harmonic movement that underlies a melody supports its push toward the moment of rest, toward the cadence.
Harmony also leads the ear toward the ultimate goal, the return to tonic.
This site covers theory from a slightly different perspective. Read this if you have a firm grasp on how we have defined cadences in class. Everything this site's author writes fits in with our understanding of cadences, he simply adds some more qualifications and categories. Cadence info from Douglas Green's Form in Tonal Music Definitions and examples taken from this informative book. You can also find this book in our library.