UL (Underwriters Laboratories) is a safety consulting and certification company headquartered in Northbrook, Illinois. It maintains offices in 46 countries. UL was established in 1894 and has participated in the safety analysis of many of the last century's new technologies, most notably the public adoption of electricity and the drafting of safety standards for electrical devices and components.
UL provides safety-related certification, validation, testing, inspection, auditing, advising and training services to a wide range of clients, including manufacturers, retailers, policymakers, regulators, service companies, and consumers.
A good nine-word summary used by UL is, "A not-for-profit organization dedicated to public safety."
UL sets standards for different product categories and tests products to make sure they meet the standards. In this way, UL is similar to the CE marking organization. UL may also certify the installation of products in the field.
Underwriters, by the way, are insurers. According to Encyclopedia Britannica, "The word underwriter is said to have derived from the practice of having each risk taker write his name under the total amount of risk that he was willing to accept at a specified premium."