In jazz, major chords (triads) are often extended to a major seventh chord form.
A G major chord 
becomes a G
major 7, 
a C major chord 
becomes a C
major 7, 
or an F major chord 
becomes an F major 7.
When these chords are held for four beats (or more), they are usually extended to a major 7th (for 2 beats) and a major 6th (for 2 beats).
The G major seven chord alternates with
the G6 chord, 
The C major seven chord alternates with
the C6 chord, 
or the F major seven chord alternates with
the F6 chord.
Minor chords (triads) are usually extended to a minor 7th chord form except when they are functioning as the tonic or I chord in a particular key.
A G minor triad 
becomes a G
minor 7, 
or a
D minor triad 
becomes a D
minor 7 
or 
.
This is the "color: chord. It has the largest number of extensions (9ths, 11ths, 13th) and alterations (sharp 9th, flat 9th, sharp 5th, flat 5th). [Altered chords are discussed in the Advanced Chords for Jazz section.] Jazz players will almost always extend the dominant 7th chord (V type chord) to a dominant 9th chord.
The D7 chord 
becomes a D9
chord: 
or 
.
The G7 chord 
becomes a G9
chord: 
or 
.
When a dominant 7th chord occurs for 4 beats, jazz players usually substitute a minor seventh chord (for 2 beats) and a dominant 7th or 9th (for 2 beats). This creates a ii7 - V7 progression.
For example, during a G7 change, the jazz
guitarist will play then .
For example, during a D7 change, the jazz
guitarist will play 
then.
The dominant 13th chord is the next most frequently used extension of the dominant 7th chord in jazz.
Two examples: 
and 
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