Basic Information
A Warship is a ship specifically intended for military use - depending on the era it may be purpose built or may be a civilian ship fitted out for combat. In general the more primitive the warship the less it will vary from a civilian vessel, but as late as WW2 auxiliary cruisers and Q-ships were using armed civilian hulls for various reasons (mostly disguise).
Where purpose built, a warship is likely to be more strongly built and better armoured than a civilian vessel, equipped with more powerful propulsion systems and (naturally) much better armed.
In any context where the combattants aren't using ordnance the distinction between military and civilian hulls is likely to be small - ships only at risk from personal weapons won't require heavily cut hulls or much armour and being better armed will usually be a function of the number of men aboard - which is usually increased by using the same sort of designs as merchants use to increase cargo capacity.
Ordnance, of course, turns these design features on their heads quite rapidly.
One particularly noteworthy piece of "ordance" is the ram. This was, essentially, a metal1 spike mounted on the front of the ship which was used to poke holes in the target. From classical antiquity until the age of steam, these things were standard issue kit (albiet mostly in the Mediterranean and a few other areas nearby) and a specific class of ship - the war galley was built around them.