Tracing wheels are usually a blunt or serrated edge wheel on a wooden or plastic handle. The blunt ones can be used for tracing with carbon paper to mark off one pattern onto another paper or piece of cardboard. The serrated or spiked wheels are the ones I see most commonly used which pierce a line of small holes through from one layer of paper or cardboard onto the layer underneath.
Tracing wheels also allow you to trace off sections of toiles which you have draped on a mannequin or on fine fabrics can be used to mark out lines and markings where needed, such as pleat placements. Always check your wheel first to avoid damaging your final fabric.
Image from PerfectPatternAids.
