A unison is a single note, whereas an interval consists of 2 notes of different pitches:
Intervals occur in 5 classes:
When inverted (turned upside down):
(Illustration of inverted intervals)
Chords occur in different qualities. Three note chords are known as triads. There are four types of triads:
Major chords (triads) consist of a stack of 2 intervals: a major 3rd on the bottom, and a minor 3rd on top:
(Illustration of major chords - notation)
Minor chords (triads) consist of a stack of 2 intervals: a minor 3rd on the bottom, and a major 3rd on top:
(Illustration of minor chords - notation)
Augmented chords (triads) consist of a stack of 2 intervals, both of which are major 3rds:
(Illustration of augmented chords - notation)
Diminished chords (triads) consist of a stack of 2 intervals, both of which are major 3rds:
(Illustration of diminished chords - notation)
Chords can be turned upside down: they are known as inversions.
Chords containing more than 3 notes are termed "extended chords", because they extend beyond the mere triad. There are many extended chords, but here are some of the more common:
Minor 6th chords
Major 7th chords
Major 9th chords (the "major" refers to the major 7th in the 9th chord)
(Illustration of extended chords)
Chords can also be altered through the use of sharps and flats. For example, a 7th chord might be altered by lowering the 5th 1/2 step. It would be notated as a C7-5 chord. Another example would be a flat 9th chord, notated as C-9.
(Illustration of altered chords)
Chords can also be used on top of a note which is not part of the chord. These are known as "slash chords" because they are notated like this: C/B F7/G etc.
(Illustration of slash chords)
In addition to standard music notation, there is a "shorthand" kind of notation known as chord symbols. They typically are found above a melody line (tune of the song) and instruct the musician as to what chord to use at any given time:
*It should also be noted that not all musicians agree on the names or symbols for all chords. There is no "authority" that passes down rules for such things; no congress that enacts laws about chords and how they are named. But there is general agreement on perhaps 90% of all chords; only on the more complex chords will you see much disagreement.
Here are some typical piano 3-note chords (triads):
Here are some typical extended chords (4 or more notes):