A warship is a ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 that is built and primarily intended for combat
Combat, or fighting, is a purposeful violent conflict meant to establish dominance over the opposition, or to terminate the opposition forever, or drive the opposition away from a location where it is not wanted or needed....

. Warships are usually built in a completely different way from merchant ships
Cargo ship
A cargo ship or freighter is any sort of ship or vessel that carries cargo, goods, and materials from one port to another. Thousands of cargo carriers ply the world's seas and oceans each year; they handle the bulk of international trade...

. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships. Unlike a merchant ship, a warship typically only carries weapons, ammunition and supplies for its own crew (rather than merchant cargo).