A chord is any set of three or more notes that is heard simultaneosly.
The most frequently encountered chords are triads (3 notes).
Chords are classed by their root note: the chord C is built upon the note C.
The most commonly used chords in western music (using the C major scale) are:
C Major, G Major and F Major (also A minor).
Many other songs can be accompanied with thischords: Ode to Joy, Twinkle twinkle little star, Oh when the saints, … but not just the classical music uses this 4 chords…