Harmony. Chords and arpeggios

Harmony is the use of different notes (pitches) simultaneously.

from http://www.bluesjazzpiano.com

Most harmony is based on chords.

A chord is a group of notes played simultaneously. The basic chord is named triad. It consist of three notes: the root (any note you like) together with the the third an the fifth above the root.

When the notes of a musical chord are played one after another instead of together, we speak about an arpeggio.

• Chord
• Arpeggio   
• Arpeggios  by J.S. Bach 

Chords can be majors and minors. 

• The third above the root is major (two tones) in major chord.
• The third above the root is minor (one tone, one semitone) in minor chords.

from http://www.piano-play-it.com

• Major chord
• Minor chord

Consonance (concord) is a nice sound that you get when some notes are played together

Consonance

Disonance (discord) is an awful sound that you get when some notes are played together

Disonance

Listening examples from Wikimedia Commons