A chord progression is a series of organised chords which make the flesh of a piece.
Figure 1: A basic chord progression over four bars.
Although a song may use chord progressions containing only its diatonic chords, non-diatonic chords from a different keys may be used to make a song sound more interesting.
Roman Numeral Analysis
When working with chord progressions, roman numerals may be added below the staff to indicate which chord is being used in relation to the given key.
Figure 2: Our chord progression again with roman numeral analysis.
Also see Chords | Cadences.