What is a chord?
A chord is a group of notes that can be played together and work as the harmony in music. There are many different chords, and they can be organized into groups and categories.
One way chords differ is by how many notes they include. There are triads, four-note chords —sometimes called tetrachords— and five-note chords. There are also six- and seven-note chords; see the detailed summary of chord types.
Building chords
A good way to learn piano chords is to become familiar with how they are built. Cmaj7 adds one note to C, the seventh in the C major scale. Cm7 adds one note to Cm, the seventh in the C minor scale.
Looking at extended chords —C7, C9, C11— shows notes being added by intervals from the root. See the step-by-step chord construction list.