A chord consists of three or more tones that sound harmonic together. The tones can be played simultaneously or one after another (broken chord). The simplest chord is a triad, so just three tones.
The most common triads in pop music are major and minor chords. For example: a C major chord consists of the tones of C-E-G. A C minor chord consists of C-Eb-G. Seventh chords are also used a lot. These chords consist of four tones and there are lots of different kinds, but in pop music, the minor and major versions are most commonly used.