At last I am starting to get my head round jazz chords. If you haven't come across jazz chords yet, try this for starters!
Play the simple chord progression C, Am, Dm, G7 a few times. Now we'll change them to jazz chords..
C Am Dm G7 becomes CM7 (also notated CMaj7) Am7 Dm7 G7
C becaomes CM7. For CM7, simply move from the C chord down to the 2nd fret. That's CM7, or C Major 7.
Am becomes Am7. For Am7, simply play all open strings.
Dm becomes Dm7. For Dm7, finger Dm but add the C with the little finger (pinky) as if playing a C chord. That's Dm7.
Then play G7 as normal.
In their simplest form, jazz chords are major 7ths and minor 7ths. And as you've just discovered, a major 7th is not the same as the usual 7th chords that we learn as part of the basic chord library. Those are actually dominant 7ths!
There are lots more jazz chords... but this'll do for an introduction!