Perhaps the most useful kind of chord is the 7th chord, partly because there are so many variations of 7ths. Far and away the most-used 7th chord is the dominant 7th, notated as F7, Bb7, D7, etc. But there are also at least 6 other types of 7th chords, including maj7, dim7, aug7, m7, etc. Watch this short video and you’ll get the idea:

Major 7th chords are formed by adding the 7th note of the major scale to a major triad. Watch the video, then go to maj7 chords for more information.