A sloop (from Dutch sloep, in turn from French chaloupe) is a sail boat with a single mast and a fore-and-aft rig comprising a mainsail and a single foresail (or headsail). By comparison, a cutter has a mainsail and at least two foresails, and its mast may be set further aft than on a sloop. The so-called "Friendship Sloop" has more than one foresail, and is, strictly-speaking, a cutter.
The commonest rig of modern sailboats is the bermuda sloop. Typically, a modern sloop carries a mainsail on a boom aft of the mast, with a single loose-footed headsail (a jib or a genoa jib) forward of the mast.
Sloops are either masthead-rigged or fractional-rigged. On a masthead-rigged sloop, the forestay (on which the headsail is carried) attaches at the top of the mast. The mainsail may be smaller than the headsail, which is then called a genoa jib. On a fractional-rigged sloop, the forestay attaches to the mast at a point below the top, typically 3/4 of the way to top, or perhaps 7/8 or some other fraction. The mast of a fractional-rigged sloop may be placed farther forward, and compared to a masthead-rigged sloop, this results in a rather smaller jib relative to the size of the mainsail. The J-24 (pictured) has a fractional rig.