During the age of sail, a frigate was a fast, medium warship with 24-60 cannons. It was used by all European navies from the mid 17th to the end of the age of sail in the 19th century. It derived from the “fregata”, which was a small oared sailing ship used in the Mediterranean sea in the late 15th century. It was smaller but faster than the largest warships, the two- and three-deckers that comprised the ships of the line.
In their hey-day they were the eyes of the battle fleets, acting as despatch vessels, passing on flagged orders during battle, and either "finishing off" or towing away as prizes stricken enemy ships. They also undertook anti-pirate ( and later, anti-slaving) duties and “showing the flag” diplomacy where the use of a full ship of the line was deemed unnecessary.
In the 1790s the young United States built a number of frigates that were the equal or better than any in commission at the time. The most famous of these, and one still afloat, is the “USS Constitution”
Since 1975, warships smaller than a destroyer and used for escort duty have also been called "frigates". In the United States Navy frigates have received the hull designations of FF (frigate), FFG (Fast Frigate with Guided Missiles) and DE (Destroyer Escort).