There are two main ways of describing a sailboat, by the boats hull type and by its rig or masts and sails. By and large, the rig is mostly used to describe larger boats and yachts and hull type to describe small boats. However there are a few exceptions where both descriptions are used.
For yachts, the vast majority of which use “fore and aft” rigs ( one or more triangular sails), the most common descriptions are sloop, cutter, ketch, yawl and occasionally schooner. These can be determined as follows:
A sloop has two sails, one extending from the bow to the mast called a jib and one extending behind the mast and along the boom ( the “spar” or pole that is perpendicular to the mast) called the mainsail. This is probably the most common type of rig.
A cutter looks identical to a sloop but the jib is usually fastened to an extension pointing forward from the bow called a “bowsprit”. Sometimes there may be more than one forward sail with the second sail usually referred to as a “staysail”.
A ketch is a very common rig for larger yachts. It will have two masts with the forward mast (the one closest to the bow and called the main mast) taller then the aft mast ( the one towards the rear of the boat and called the “mizzen”). The aft mast will be mounted forward of the “rudder post” or where the rudder is mounted to the boat. A ketch may or may not have a bowsprit and may or may not use one or more staysails.
A yawl is not as common as a ketch. The description of a yawl is the same as that of a ketch with two exceptions. The mizzen mast is mounted behind or “aft” of the rudder post and it is usually significantly smaller then the main mast.
A schooner is not a very common rig these days. It two is similar to a ketch except that the masts are reversed. The smaller mast is up front and called the “foremast” and the aft mast is the same height or much taller and called the main mast. In a schooner, the main mast is usually mounted more to the front of the boat then it would be on a ketch. A schooner may have more than two masts, may or may not have a bowsprit and often uses staysails and other types of sails.
A catamaran will have two hulls with a deck housing between them. They usually come rigged as a sloop or ketch. A trimaran is the same but with three narrow hulls.
In small boats the vast majority of the rigs are sloop so these boats are usually described by the hull type.
A dinghy is usually a small boat that is mostly or completely open.
A scow is usually a very “flat” wide hull that is mostly open and low to the water.
A cruiser is a somewhat larger heavier boat with high sides or “freeboard” that will have a small cabin.
A catamaran will have two very narrow hulls, usually with a fabric deck or “trampoline” between the hulls.