A cadence (or close) is a harmonic or melodic progression which ends a phrase, section, or composition.
There are five different cadences: perfect authentic, imperfect authentic, plagal, imperfect, and interrupted.
Perfect Authentic Cadence
A perfect authentic cadence is the motion of the V chord to the I chord with both chords in root position:
Figure 1: A perfect authentic cadence.
Imperfect Authentic Cadence
An imperfect authentic cadence is the motion of the V chord to the I chord with one or both chords not in root position:
Figure 2: An imperfect authentic cadence.
A plagal cadence is the motion of the IV chord to the I chord:
Figure 3: A plagal cadence.
An imperfect (or half) cadence is the motion of any chord to the V chord:
Figure 4: An imperfect cadence.
An interrupted (or deceptive) cadence is the motion of the V chord to the vi chord:
Figure 5: An inturrupted cadence.
Also see Chords | Chord Progressions | Counterpoint.