Uninterruptible Power Supply (UPS) terminology is a complex subject, often made more confusing by the competing marketing claims of various manufacturers. This can make it difficult for specifiers and owners to understand and evaluate the options provided by competing vendors. The North American industry should consider the approach and terminology developed by the IEC. The IEC has defined UPS types based on the relationships and dependencies (or lack thereof) between input and output voltage and frequency. This paper explains these terms and discusses the capabilities and limitations of the various classifications.