A cadence is a harmonic progression that comes at the end of a phrase. A cadence is often marked by metric accent on its final chord (that is to say the final chord of the cadence often falls on the downbeat of the next measure). As well, harmonic rhythm typically accelerates before a cadence. It comes in a few varieties:
Perfect Authentic Cadence: A perfect authentic cadence goes from V (or V7) to I with the soprano (or otherwise highest) voice sounding the tonic of the I chord. In a perfect authentic cadence, both chords are in root position. The perfect authentic cadence usually comes at the end of a piece and is often preceded by the cadential V6/4 chord which often resolves to a V7 and then goes to I. This cadence is analogous to a period and usually appears at the end of an antecedent phrase.
Imperfect Authentic Cadence: An imperfect authentic cadence is the same as a perfect authentic cadence except that the highest voice sounds the third or the fifth of the final I chord. Again both chords are in root position and often the V chord is preceded by a 6/4-5/3 linear motion. This cadence is analogous to a comma or a semicolon.
Half Cadence: A half cadence is any phrase ending on V. It is usually metrically accented and the V chord usually sounds for a longer duration than the chords leading up to it to preserve a cadential feel. In minor, a phrygian half cadence is common and is marked by the motion of a IV6 (first inversion) chord descending by step in bass to a root position V. The half cadence, like the imperfect authentic cadence can be analogized to a comma or semicolon and usually appears at the end of an antecedent phrase. It is important to note that a half cadence never ends on V7 since the chord contains the dissonant tritone and is therefore unstable.
Plagal Cadence: While a plagal cadence is not a real cadence it is worth noting. A plagal cadence consists of the motion of root position IV to root position I in the same fashion as any other cadence. Typically, a plagal cadence is used to expand tonic harmony (i.e. extend the duration of a tonic chord and the stability that it represents) or at the end of a composition, directly following a perfect authentic cadence to create an even more cadential effect. The classic example is the "amen" at the end of a prayer which goes IV-"a", I-"men".