This image here
http://www.flickr.com/photos/scottdukes/316721609/ ...But i wanan take it at night...
oh, this technique is called panning.
dosent matter if its in the day or at night.
however, u need a relatively slow shutter speed for the backgrd blur, and not too slow such that there is visible handshake. (like 1/20 - 1/40? depends on how fast moving the subject is also) so u basically set ur camera to shutter priority.
a few mtds of focusing on the subject.
1. pre-focus on an area in which the subject would be. (like if u want to shoot a moving car that will be driving over say lane 2 of the road, u use AF to focus lane 2, and set to MF to stop AF when u are shooting)
2. use AI servo focusing mode (on canon cameras that is; im not sure abt the term used on other brands of camera). this would auto focus constantly on a subject that is always moving.
then, as the car comes, u press the shutter button, and follow thru the motion of the car while the shutter is open.
this will result in a sharp moving object (since u are following its motion, thus relative to the frame, the car dosent seem to move) while the stationary backgrd will be blurred relative the frame. u need a slow enuff shutter to capture the motion blur.
requires some practise. have fun!