Hi, Ive been doing Parkour for about a year. Its great for fitness. Id reccommend it because it helps you to think seperately from society. You can run, instead of walk. You can saut de chat (cat jump) that fence instead of go through the gate. Don't get too stuck on reading stuff about it. Get out there and run. One thing first. Learn how to roll. This can help prevent injury and is the basis for many other inportant moves, as it aids in co-ordination. Learning on concrete is no biggie, just only do jumps and stuff. Learn rolls on grass.
If you want info on Parkour, go to
www.parkour.net it really is the most authorative source.
BTW, there are two main branches of parkour; Free style (for show ponies and trixters) and free running (all about fluidity, speed and purpose [this is the one that i do] )
If you want to do some research, David Bell was the creator of Parkour and people that practice it are called traceurs and traceuresses. But just get out there and start.