The UL stands for "Underwriters Laboratory". It used to be

an Insurance Industry organization, but now it is independent

and non-profit. It tests electrical components and equipment

for potential hazards. When something is UL-listed, that means

that the UL has tested the device, and it meets their requirements

for safety - ie: fire or shock hazard. It doesn't necessarily

mean that the device actually does what it's supposed to, just

that it probably won't kill you.

The UL does not have power of law in the U.S. -- you are

permitted to buy and install non-UL-listed devices. However,

insurance policies sometimes have clauses in them that will

limit their liability in case of a claim made in response to

the failure of a non-UL-listed device. Furthermore, in

many situations the NEC will require that a wiring component

used for a specific purpose is UL-listed for that purpose.

Indirectly, this means that certain parts of your wiring

must be UL-listed before an inspector will approve it and/or

occupancy permits issued.