In general there are 5 different types of 7th chords that are often played by guitar players in different genres. Other than the three we are discussing in this post,  the two other 7th chord shapes are called half diminished chord (also known as minor seventh flat five chord) and the minor major 7th chord.

Major 7th: It is a major triad with an added major 7th. In the key of C it is C  E  G  and B

Minor 7th: It is a minor triad with an added minor 7th. In the key of C it is C Eb G and Bb

Dominant 7th (also known as 7th): It is a major triad with an added minor 7th. In the key of C it is C E G and Bb

The unique characteristic of the dominant 7th chord is that it contains a tritone (E and Bb) that gives the chord a very strong tonality. The other interesting thing about this chord is the blend of major and minor intervals. This allows the musicians to play both major and minor intervals on top of the dominant 7th chords, which is very common and popular in blues.