'Parkour is a discipline, were one learns to interact with their enviorment.
Freerunning is the application of this discipline as a sport.
It is always moving forward, never stopping to look back.'
This is what me, and my friends, tell anyone who asks what the hell we're doing.
Also, I personally consider Freerunning to simply be a combination of Parkour, learning to move through your world, and Tricking, learning to use momentum and move your body in all ways possible.
Of course, I'm just a rookie, so don't take anything I say seriously.