The KT88 audio tube is considered to be one of the best sounding power tubes for hifi tube audio amplifiers. In general, the KT88 tube has a magical mid-range similar to the smaller EL34 tube, but with low and high end extremes that reach much further. The KT88 was invented by Marconi-Osram Valve Co. and became quick competition for the USA made 6550 tube. Companies including Leak, McIntosh, Dynaco, and Harmon-Kardon all offered high-end amps that would take advantage of the KT88 tube's abilities. Marshall also got into the action a few years later when they released the 200 watt Marshall Major amplifiers.
Be sure to click the “Additional Info” tab when viewing a specific KT88 / KT90 / KT120 tube. There you will find related articles and downloads such as datasheets, pin-outs, ratings, and matching information when available. Other excellent links for technical information can be found at KT88 in Wikipedia, KT88 in Duncan Amps, and KT88 in The Tube Directory.