Magnetic signs are useful when you don't want to put vinyl lettering and graphics directly on your vehicles paint. They can be removed easily when you don't want to advertise. Most magnetic signs are vinyl lettered with 1 or 2 colors, and some are printed in full-color. Standard sizes are 12x18" and 12x24", but can be cut to any size to fit your requirements.
Magnetics are also used for other applications other than vehicle advertising such as machine markers, bin indentification, bus numbering, etc.