How To Start With Parkour

First of all, you must know what Parkour is. Parkour is fastest way to get from point A to point B using possibilities of human body. Parkour isn't about running on rooftops or jumping from high places. OK, so you saw someone on some video doing that. It was probably David Belle, Yamakasi or some other skilled traceur (traceur is a person who train parkour). You must know that they trained very hard for a long time to be able to do that. And yeah, flips and acrobatics aren't part of parkour.
Visit website www.parkour.net and read all the articles. They useful and very well written. There is also forum so if you want to ask something, just ask there.

TV report about Parkour, David and Yamakasi