#1
In what styles of music would you use major 7 chords? are they the main chords or just there to spice up regular chords?
#2
any style of muisc where it fits (if your asking where its been used: jazz, funk, classical, more adventurous pop and rock music). you'd use them any time youd use a major chord that isnt a dominant chord (diatonically they occur as the I and IV chords), a major seventh chord a half step away from the tonic can also have a pre dominant function, as it leads to a V chord in a similar manner to a iim7 chord (for example: DbM7-G7-C) .
all the best.
(insert self-aggrandizing quote here)
#4
Quote by Sean0913
I call em "Love song chords" but Jazz uses them a lot. Its a very pretty chord.

I also tell students, that if your girlfriend ever asks you to compose a song on the spot, just while looking at her, whip out a GMaj7 to Dmaj7 and you're home free.

Best,

Sean
http://www.youtube.com/watch?v=ULrz-6CSmmM
Only play what you hear. If you don’t hear anything, don’t play anything.
-Chick Corea