Twin needle employs a chain stitch, single needle a lock stitch (conventional top and bobbin thread). Look at the diagrams here: http://home.howstuffworks.com/sewing-machine.htm
Have a look at the inside leg seam of a pair of Levis. That is a twin needle seam. Both fabric edges are folded over and then a twin needle machine, utilizing a chain stitch goes over the lot. Do you feel the roughness of the underside? That is where the thread forms a loop. A single needle seam is equally stitched twice but with a flat felled seam. One row of stitching is hidden inside the fabric. From the topside you see only one row, but you see two rows of stitching from the underside. http://www.backpacking.net/makegear/...latfelled.html
Twin needle seams are less labor intesive, but more prone to puckering. Most American shirts are single, most European ones, bar the top makes, are twin needle. Have a look at a Charvet shirt to see single needle at it's finest.