Light bulbs that require a direct line voltage to power them don't need an exernal power supply. These lights have a quick and easy installation, and dont require a transformer. These light fixtures are often referred to as plug-and-play.  Line voltage is the standard 120 volts -- that's found in circuit boxes and outlets.  Low voltage lighting, on the other hand generally uses 12 or 24 volts and requires a transformer. Always be sure to determine the maximum amperage for your ciruit to determine the amount of lights that you're  allowed to run on one circuit.