Major 7th chords have a jazzy, breezy feel. They are settled, and sound nice. (Spelled 1,3,5,7)
Dominant 7th chords, spelled 1,3,5,b7 (eg E7, A7, B7) have a tension that wants to go somewhere. But here's the interesting bit. C7, which is C,E,G, Bb sounds tense. Why? Because hidden in there is a b5, the tritone, an interval banned in the middle ages because of it's evil dissonant quality! It is the interval between the E and the Bb. It is hidden in the chord, but you still hear it.
Can also be useful to think of "major" as "natural."
That b5 interval hidden in the dominant 7th chord will be useful to those who didn't know it already. Just wait and see.
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