Heres some good reading on double clutching http://integra.vtec.net/driving/dclutch.html

But MKerley had it right about the definition of double clutching. Clutch in > put car into neutral >clutch out >clutch back in>car into gear of choice > clutch back out. Syncros in your tranny do all the work for you so you can just go from gear to gear, and the syncros will match speeds for you so you dont get the not so fun "crunch" ;) But in older cars without syncromesh transmissions, you have to double clutch to get your gears to engage cleanly :)

- Ryan