Frigates were three masted, medium sized square rigged warships. They were built for both speed and maneuverability. Depending on how many guns and gundecks they have, they're divided in rates.
History[]
The Dutch Navy became the first navy to build large, ocean going frigates. They were built especially to both have sufficient armament that they could go toe to toe with the Spanish Navy, and to be fast and shallow enough that they could maneuver around the shallow waters around the Netherlands.
The HMS Milford, a Royal Navy frigate.
Eventually, England began building frigates that were third rate ships of the line, meaning they carried 60 guns.
Depending on the armament, frigates generally had one or two gun decks, armed with 28-60 guns. They were used for escort duties and reconnaissance.