A cadence is the ending to a musical phrase, the period to a sentence.  Cadences are usually comprised of two chords, sometimes more, and they appear after major points in a musical piece.  If you have ever sung a church hymn, you've heard countless numbers of cadences, at the end of a hymn, or the popular AMEN.

The Classical Cadences

Authentic Cadence
    - Chord Progression V to I

There are two types:
    - Perfect Authentic Cadence - when top note ends on the tonic (no inversion)
    - Imperfect Authentic Cadence - when top note does NOT end on the tonic

Half Cadence
    - Chord Progression that ends on V (I to V) (IV to V)
    - Usually used when modulating to the dominant key

Plagal Cadence
    - Chord Progression IV to I
    - Often refered to as the "Amen Cadence"

Deceptive Cadence
    - Chord Progression V to vi
    - called "deceptive" because the listener expects V to I, but hears V to vi
    - used to prolong a passage

Six-Four Cadence
    - Analyzed as a V to I progression, but actually is a I6/4 to V to I progression
    - contains three chords