I have seen pictures, for example a city at night, and all the traffic on the road front and back headlights are all blury, but the buildings and scenery are crystal clear. How is this done??? Thanks
It's quite simple really, the camera has to be perfectly still. The blurriness is a result of movement that the camera captures with a long shutter speed, but anything that doens't move, won't be blurry. If you are holding the camera in your hands, there will be some small movement, which would cause everything to be blurry...but if you use a tripod along with a remote release (or self timer), then the camera won't move and you can get the desired result.
Another bit for you, if you want some color in your photo aside from the lights, take the shot just before it gets completely dark. If you have a digital and the time, take a shot every 5 min before it gets dark and then every 5 min after for a 1/2 hour either way and you'll see when you like the best for future reference.
Most low-end DSLRs support up to a 30 second exposure, which would plenty to get the kind of headlight/tail-light blur you're wanting from the moving traffic. Some cameras can achieve 30 minute exposures or longer, with which you would be able to not only see the blur of the headlights, but the blur of the stars.
You will be doing yourselves a HUGE service if you acquire at least a passing understanding of Ansel Adam's Zone system. You can find 'lite' versions on the net that will do fine for most applications.
To get a correct exposure with your camera's meter you need to meter off something that will approximate medium grey and then fine tune by "bracketing' the medium grey with the zone system to properly expose the image to your desired vision.
All this is to say that with the zone system you can easier guesstimate to get the shot to look like you want it.