In military terminology, a cruiser is a large warship capable of engaging multiple targets simultaneously.
Modern United States Navy guided missile cruisers (CG and CGN hull classification symbols) perform primarily in a Battle Force role. These ships are multi-mission -- anti-aircraft (AAW), anti-submarine (ASW), and anti-surface (ASUW) surface combatants capable of supporting carrier battle groups, amphibious forces, or of operating independently and as flagships of surface action groups.
Technological advances in surface-to-air missiles coupled with the Aegis combat system have increased cruisers' AAW capability, while Tomahawk missiles give them long-range strike mission capability. These weapon systems have restored an offensive strike role to the surface forces that seemed to have been lost to air power at the battle of Pearl Harbor.
All cruisers currently in commission in the US Navy are members of the Ticonderoga class.
Historical subtypes include armored cruiser, protected cruiser, auxiliary cruiser, heavy cruiser, and light cruiser. The battlecruiser could be considered a sort of hybrid between cruiser and battleship.
The term "cruiser" was a mid 19th century invention. When warships were made of wood and had sails, frigates were small, fast, long range, lightly armed (single gun-deck) ships used for scouting and carrying dispatches. The first ironclads also had only a single gun-deck because of the weight of armor, even though they were bigger ships with bigger guns. They were nevertheless referred to as frigates although they were really ships of the line. Thus the definition of a frigate changed.
Ships which carried out the original frigate role were now renamed "cruising ships", which was rapidly abbreviated to cruiser. This continued to be the meaning until after the Second World War - a fast, long-range, lightly armored ship, although by then more powerful than a destroyer.