Civil War period ship terms can be confusing to the casual observer.  To make matters worse, the very nature of warships was changing during this period.  This was largely due to the coming of the steam engine, which allowed operation in any weather and in coastal and river environments previously untenable to warships.  It was also due to the introduction of iron and steel hulls and armor which, at this point in history, went to see alongside ships unchanged from the days of Nelson. 

Modern navies often classify ships by the mission they intend to undertake.  Civil War period ships were classified by their rate, or how many guns they carried.  Thus the term cruiser in the civil war is used to describe any ship assigned to ship cruising with out a station and on its own and has nothing to do with size or equipment.  Naturally this list features assumptions to make a general rule of thumb that can be used to equate period vessels to modem ones.  Certain comparisons will be found not to hold true as naval warfare now is drastically different then in the 1860's

• Ships of the Line = Battleship or Carrier  Ships of the line were classed during this period as carrying 74 guns or more.  They usually had two complete gun decks, and a partial third.  The US never operated many Ships of the line, as they were expensive.  Most served as training ships, or spend their lives in storage.  The largest, the 120 Gun, Pennsylvania was burned at the Norfolk Navy Yard early in the war to prevent capture.  The US never had steam powered Ships of the Line.
  
• Frigates = Cruisers  The Frigate at this time ranged between 32 and 50 guns.  The US Navy was always centered on the Frigate as it was flexible in mission and cheaper then Ships of the Line.  US Frigates were larger then their counterparts in Europe and generally better armed.  The US had a collection of Sailing Frigates, and five 50 Gun Steam Frigates at the start of the war (one of which, the Merrimack, was burned at the Norfolk Navy Yard)
  
• Sloops of War = Destroyers  These vessels were divided into two classes.  The First Class were fitted with broadside guns and traditionally fitted out, examples being the USS Hartford, or USS Constellation.  Second Class featured large guns in pivoting batteries, and example being the USS Kearsarge. 
  
• Gunboats = Frigates or Patrol Craft  The use of the modern term Frigate has been applied to the class that in World War Two was known as the Destroyer Escort.  Gunboat rose to favor as a term, over the former Man of War, with the coming of steam power.  Many gunboats were converted merchant vessels, while others were smaller copies of the Second Class Sloop of War
  
• Ironclads and Monitors  In the cases where ironclads were built on the traditional scale, an example being the USS New Ironsides, which was classed alternately as a Sloop and a Frigate. Monitors were outside traditional naval experience and were viewed with distrust by experienced sailors. They formed a niche of their own in the fleet which today would be hard to describe, but might best be compared to serving in a Submarine