Q: Hold on I'm confused. I get that maj7's are the root + third + fifth + seventh notes in a major key (at least I think), for example Cmaj7 = C E G B. But then what are the 7's?
A: C7 is actually an abbreviation for 'dominant 7th' don't ask me why it's called dominant, i suspect it was popularized by a person named dominant. Or you could think of it as an abbreviation for C(b7)