Hi everyone, Kevin here! A basic major or minor triad contains the 1st, 3rd and 5th scale degrees.
For example C major is C - E - G, follow the logic one more step and add the 7th scale degree and you have a 7th chord.
C - E - G - B is a Major 7th chord, and has a very melancholy or jazzy sound. Use of the 7th scale degrees as well as the 9th, 11th and 13ths are essential in jazz.
C - E - G - Bb
Now lower that B down to a Bb and you have the signature sound of the blues. The chord that has the gravitational pull to resolve down to the tonic chord. This is also known as the dominant 7th chord.