Light trails are another way of utilising lights in a long exposure. A popular use of this technique is to take photographs of cars and traffic. This can give an interesting element to the photograph. It's important that the traffic keeps flowing by during the course of the shot. If it stops, you may get cars showing up in your photograph which you don't want as this may ruin the image.
Sometimes, instead of gathering all the trails in just one image, people use photo stacking. This is where you take multiple photographs of the traffic flowing buy. Each photo will likely only have a fairly small trail but you can combine these images on the computer to create a series of much longer trails. This is usually done using a "lighten" blend in Photoshop. Here are some examples for you:
18mm 15s F1.8 ISO 100
22mm 1.6s F8 ISO 100 - Luna Park, Sydney, Australia
26mm 10s F1.8 ISO 100
35mm 13s F16 ISO 100 - Lane Cove Tunnel, Sydney, Australia
18mm 30s F6.3 ISO 100 - The Domain, Sydney, Australia
35mm 15s F8.0 ISO 100 - Porchse Willoughby Centre, Sydney, Australia
There's not much more I can think of. The hardest part of light trails, I find, tends to be just finding the right locations. So next you're out and about, see if you can note any places where you can set up for a shot like this. Then once you find it, just set up your tripod and set a long shutter speed.