A Ship is a big boat. Usually a boat large enough to carry it own boats, like lifeboats, dingies or runabouts. A rule of thumb saying (though it doesn't always apply) is "a boat can fit on a ship, but a ship can't fit on a boat. The exact size at which a ship becomes a boat is often defined by local law.
In nautical history terms, a ship specifically signifies a three-masted sailing vessel with square sails on all three masts.