Dominant 7th chords add a minor seventh harmonic interval to a major triad. A major seventh chord adds a major seventh harmonic interval to the major triad. Despite the fact that these chord types share the heritage of the major triad, they mean entirely different things to listener's ears.
	The major seventh feels fairly restful because the harmonic intervals formed between notes of the chord are pretty consonant. Between the root and perfect fifth and between the major 3rd and major 7th, intervals of the perfect 5th are formed. Between the Root and major 3rd and the fifth and the major 7th, major 3rds are formed. These are the most consonant harmonic intervals used in music. The major 7th actually adds an edgy feeling to the chord, but it usually can't overwhelm the other consonances that sound in this chord.
	The dominant 7th feels unsettled and restless. Between the major 3rd and minor 7th in the chord, the harmonic interval called the flatted fifth is formed. In the Middle Ages this interval was called the devil in music and was banned from ecclesiatic music. The interval between the perfect 5th and minor 7th is a minor 3rd, which also has a dark unsettled feeling. Dominant 7ths are rarely places in a chord sequence where the melody pauses. When a 7th is playing, that's usually the place where the melody is most active and also the place where the more unusual harmonic intervals are formed by the melody notes. They are the points of maximum tension in a chord sequence and almost always are followed by a chord which relaxes the tension. The tension relaxing chords are frequently a chord from the major or minor familes whose root is a perfect 4th above the root of the dominant 7th chord.
	The one exception to this is that blues tunes are almost always built predominantly using dominant chords. Blues tunes move from one tense dominant 7th chord to another. The objective of a blues is to allow the performer to invoke blue notes, special harmonic intervals which were banned from Western music hundreds of years ago. Playing two dominant 7th chords whose roots differ by a perfect 4th in the certain pattern used in blues, provide a support that make it easy for the performer to find and the listener to hear these blue intervals.