how do you do this? (photography)?

It's called "Panning" and is done by following the subject at the same time (so you move the camera with the car, should use a tripod), and use a somewhat slow shutter speed (1/100 or slower)

How to pan: http://www.youtube.com/watch?v=sJcjzcO8Zts&feature...

Using a slow shutter speed, something like 1/60 sec or less.... Face away from the direction the car will be travelling, then keeping your feet firmly planted wind your torso up until you are facing toward the car.... Unwind as the car passes taking your shot while moving... This will help minimise the up and down movement of the camera. remember to keep your feet still.

You'll need to practice but once you get the hang of it, it'll be a useful tool in the box, so to speak!

it takes practice but as you take the photo, you pan across and follow the car. then the car will be in focus because you are following it and not the background.

simple answer is: PANNING

you move with the object with the camera...

http://www.flickr.com/photos/martinwalker/45488326...

http://www.flickr.com/photos/martinwalker/45481980...