A yacht is a vessel used for pleasure or recreation (as opposed to a working boat). There is a lot of variety in what constitutes a yacht (a small sloop vs. a megayacht, for example). Yachts may be powered by motor or sail, which are further sub-classified by use or amenities.
Size is the major factor in determining the class of a yacht. Vessels larger than 100 feet in length are considered mega yachts and those larger than 200 feet are generally called super yachts. As larger and larger yachts are being designed and constructed the term giga-yacht is also being considered for use.