In the modern sense, an armored warship is a naval vessel, partially or fully covered with metal armor as a passive defense against gunfire. Examples of the type began to come into service in the mid-19th century, although metal armor alone was obsolete as a defense against guided missiles. Very few were built after World War II and none are in service.
Modern warships may include armor, typically ceramic or composite, as part of their overall protection scheme. Today's doctrine, however, is to avoid being hit rather than surviving a hit.
Although Korea had armored "turtle ships" before steam, the practical definition assumes mechanical propulsion, possibly with supplementary sails. One pioneering design was the British HMS Warrior (1860), preserved as a museum ship today. Warrior was a seaworthy, ocean-going ship. Her guns were arranged along her sides, to be used in the same sort of short-range broadside firing as was the standard for sailing warships.
At the 1862 Battle of Hampton Roads, the CSS Virginia, an improvised Confederate ironclad, first showed her dominance over unarmored warships. On the next day, however, she faced the very definitely unseaworthy USS Monitor, who still had much more advanced technology: an armament of two very heavy guns in a rotating turret, and thicker armor. They fought to a draw.
Gradually, the broadside design went out of favor. It was imposed, in large part, by the limitations of steam propulsion, and the fairly primitive cannon available. As longer-ranged cannon, still direct fire, became available for steamships, movable guns offered much more tactical flexibility.
Armor is very heavy. Warship designers have always faced tradeoffs between, at the least, offensive armament, speed, and protection. One design tradeoff was to armor only the most vulnerable parts of a vessel — protect the control stations, engines and perhaps guns from the heaviest fire, but accept vulnerability of berthing spaces and other areas not essential to combat.