Double Clutching?

What exactly is the "definition" of double clutching. I once was told that you put the clutch in as you release the gear, then release the clutch, put the clutch back in, put the car in gear, and then release the clutch again... This seems like a lot of work for something that I believe is for performance??? Any knowledge that you would be willing to pass on would be great. There seems to be quite a few people who know there stuff in this forum.
Thanks

thats exactly what it is. you double clutch to bring your gear luster rpm up or down to match your engine RPM's. usually only have to do this when you have a stick that has no syncros. most Semi- Trucks have to double clutch. Also a true Hi performance race trans does not have syncros in them. they take mmore HP to spin. but are a pian in the a$$ to drive

Much appreciated.

no problem, luster in my last post was suppost to be cluster.

Well umm yeah double clutching is really unnnecessary. And semi-trucks don't even have to use the clutch to shift. I drove one a couple of times just for fun and the guy told me that the only time I have to use the clutch is at the start and in semi's you can let the clutch out pretty much all the way and they won't stall. Don't pay attention to that fast and the furious bullshit. Double clutching is not necessary in any new car. The only time where it had to be done all the time is in older cars like 60's or earlier I believe where it actually was a must. So no need to double clutch, ok to learn very easy of course, but not necessary. Double clutching is most important only when downshifting. A typical transmisson has two shafts, one connected to the engine through the clutch, and one connected to the rear wheels. There are usually four to six sets of gears on these shafts and they are selected with the gearshift lever inside the car. When changing gears, the clutch is depressed to disconnect the engine from the transmission so there isn't any stress on the moving parts. Since the output shaft is permanently connected to the rear wheels, the only way to match the speed of the two shafts is to use the throttle to adjust engine speed.
Once the engine is turning the right rpm, both shafts will be turning the same speed, and the gear lever will fall into gear WITHOUT using the clutch! (Although most of the time you are shifting too fast to be that accurate).