Destroyer

A destroyer is a warship. It is similar to, but is smaller and costs less than a cruiser, but larger than a frigate. It is usually used as an escort in battle groups.

Roles
Destroyers are used to defend larger warships, amphibious, and merchant ships. Destroyers also go independent and bombard enemy shores, participate in search and rescue missions at sea, and support amphibious landings. And as said above, it is usually used as an escort in battle groups.

Armament
Destroyers are usually armed with guns, missles, rockets, and torpedos. In rare cases, they have helicopters.

Destroyers in Navies
With only 4 nations operating cruisers, and no navies operating battleships (even though the U.S. maintains two Iowa-class battleships, just in case they need to be re-activated) destroyers are the largest general-use warship (aircraft carriers have one use, carrying aircraft) in commmon use.