Products that are UL-approved carry the "UL Listed" mark and have passed government-regulated safety tests performed by Underwriters Laboratories. The mark is most often seen on electrical equipment to show that it meets the required safety standards.
Underwriters Laboratories (UL) is a government-approved company that carries out testing and certification to ensure products meet safety specifications. These standards are based on probable hazards associated with the product. UL vigorously tests a sample of the product to determine how it reacts in each hazardous situation. If the manufacturer has proven it can create products that meet UL's stringent safety requirements, it gets approved to carry the UL Listed mark.