Yacht lengths generally range from 20 feet (6 m) up to hundreds of feet. Luxury crafts smaller than 40 feet are more commonly called "cabin cruisers" or simply "cruisers." A mega yacht generally refers to any yacht (sail or power) above 100 ft (34 m) and a super yacht generally refers to any yacht over 200 ft (70 m). This size is small in relation to typical cruise liners and oil tankers.
Yacht was originally defined as a light, fast sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into the shallow waters of the Low Countries. They were also used for non-military governmental roles such as customs duties and delivering pilots to waiting ships. The latter use attracted the attention of wealthy Dutch merchants who began to build private yachts so they could be taken out to greet their returning ships. Soon wealthy individuals began to use their 'jachts' for pleasure trips. By the start of the 17th century 'jachts' came in two broad catergories- speel-jachts for sport and oorlog-jachts for naval duties. By the middle of the century large 'jacht' fleets were found around the Dutch coast and the Dutch states organised large 'reviews' of private and war yachts for special occasions, thus putting in place the groundwork for the modern sport of yachting. Jachts of this period varied greatly in size, from around 40ft in length to being equal to the lower classes of the ship of the line. All had a form of fore/aft gaff rig with a flat bottom and lee boards to allow operations in shallow waters. The gaff rig remained the principle rig found on small European yachts for centuries until giving way to the 'Bermudan sloop' rig in the 1960s.
Charles II of England spent part of his time in exile during the period of the Commonwealth of England in the Netherlands and became keen on sailing. He returned to England in 1660 aboard a Dutch yacht. During his reign Charles commissioned 24 Royal Yachts on top of the two presented to him by Dutch states on his restoration. As the fashion for yachting spread throughout the English aristocracy yacht races began to become common. Other rich individuals in Europe built yachts as the sport spread. Yachting therefore became a purely recreational form of sailing with no commercial or military function (see, for example, the Cox & King yachts at the beginning of the 20th Century), which still serves a broad definition of both the sport and of the vessel.
source: wikipedia
