In music theory, a cadence is two chords which create a sense of closure, or rest to a phrase, section, or entire piece of music.
The most commonly used are: perfect authentic, imperfect authentic, plagal, deceptive and half cadence.
Some of the above are US-english terms. In the UK, authentic cadences are called perfect cadences, half cadences are called imperfect cadences, and deceptive cadences are called interrupted cadences.
Each of the above cadence types use different chords (or inversions) to create these rest / closure effects.
Strong cadences give a real sense of finality, and so are most often used right at the end of a piece.
In contrast, weak cadences are less conclusive, which can be used to create a sense of rest, or even surprise the listener with a false ending, when a strong cadence was expected in its place.
Cadences use Scale Chords (see Scale chord) of the appropriate Major or Minor scale.
An authentic cadence moves from the dominant (V or V7), to the tonic (I) scale degree.
The plagal cadence moves from the subdominant (IV), to the tonic (I) scale degree.
The half cadence moves from any diatonic scale chord eg. supertonic (ii), to the dominant (V) scale degree.
The deceptive deceptive cadence moves from the dominant (V), to a chord that is not the tonic(I) - eg. supertonic (ii) scale degree.
The example below shows the E-Flat major perfect authentic cadence, moving from chord V of the Eb-Major scale(Bb major) scale to chord I (Eb Major).
It is a perfect cadence as the tonic is the highest note of the chord, hence the 8th scale note being shown (the octave of the tonic).
More detail of each cadence type / subtypes are described in the cadence links above.