In western tonal music we tend to use normally four types of
three-note chords, plus a couple of four-note chords. There are
more also, but these are the most widely used.
Three-note Chords
Three-note chords (called triads) have a characteristic sound to each
type, based on the interval spacing between the three notes in the
chord. The spacings are as follows: (see Music Theory /
Intervals for a discussion of intervals)
Chord
type
lower
interval
upper
interval
Major
M2
m2
Minor
m2
M2
Diminished
m2
m2
Augmented
M2
M2
So, a major chord with a bottom (root) note of C would contain: these
notes: C, E, G
A minor chord built on a bottom (root) note of A would contain these
notes: A, C, E.
A diminished chord built on a bottom (root) note of B would contain
these notes: B, D, F
an augmented chord built on a bottom (root) note of C would contain
these notes: C, E, G#
The note names need to be two letters apart. In the last example,
you say C, E, G#, not C, E, A-flat.
These chords don't have to appear with the root note always on the
bottom. When they do, that's called 'root position'. But
when the middle note in the definition appears on the bottomm, that's
called 'first inversion.' And whtn the top note in the definition
appears on the bottom, that's called 'second inversion.'
Four note chords
The most popular four note chord is called a seventh chord.
Normally it's made of a major chord plus a note that is a minor
seventh, or sometimes a major seventh, above the root. For
example,
G, B, D, F
is a G major chord with a minor 7th above it, called Gm7, where the
capital G stands for major, and the m7 is the minor 7th above.
There are also 9th chords, with a major or minor 9th above the root
note. Jazz guitar players keep mentioning flatted 13th
chords..... which would be an octave plus a sixth above the root.
These are the basic types of chords available. But the next step
is to understand how people commonly use them in a piece of music.