The Underwriters Laboratory (UL) was set up around 120 years ago in the USA as an independent facility to test the safety of new products and new technologies. Today it has a network of sites around the world focused on product safety. It test products, certifies manufacturers, and produces and updates safety standards across a broad range of industrial and commercial sectors. More.
For PCB’s the main standards are UL 796, the specific PCB standard, and UL 94 for flammability testing of all plastics. These specify a number of performance tests to measure the long-term reliability and fire safety of the PCB. If a board is released to these standards, it is marked with the UL logo, the fabricator’s logo and the board type.
For more information please see What About UL? on the Eurocircuits website.