The maj7 is a major chord with an additional major 7th. You can get the maj7 by going up the C major scale all seven tones. Or you just go down a half-step (1 fret) from C.
The Cmaj7 has the notes C (1), E (3), G (5) and B (maj7).
The maj7 chord is a diatonic chord of the major scale. It can be assembled by stacking thirds of scale own tones on the 1st (Imaj7) and 4th (Ivmaj7) scale degree. In case of a C major scale that results in a Cmaj7 (C E G B) and an Fmaj7 chord (F A C E).
In Jazz and Soul orientated music (Soul, Funk, R&B,…) and beyond, where plain major and minor chords sound kind of shallow, maj7 chords are omnipresent. In those musical styles practically all major chords on the 1st or 4th scale degree are extended with a maj7 (or modified/extended in other ways like sus2, add9 or as 6th chords).