double de clutch is when you are changing gears in old cars:- you dip clutch, take out of gear, raise clutch, dip clutch and then rev match and put it back in gear then raise clutch!!
Basically it's when cars didnt have a synchro so you had to match the revs to the gearbox speed thing (im not mechanically minded!), if you didnt it would crunch the box and not go in gear.
Rev match (with syncro on modern car) is just matching the revs with the gear box speed (not sure if that's the right way of saying it) so that is smoother going in gear and the car doesnt lurch from having to cope with a lower gear and hgher rpm.
With the latter you dont have to do this to change gear, the former you had to double de clutch to change