Extract from the Royal Navy Articles of War, 1795
The officers and seamen of all ships appointed for convoy and guard of merchant ships, or of any other, shall diligently attend upon that charge, without delay, according to their instructions in that behalf; and whosoever shall be faulty therein, and shall not faithfully perform their duty, and defend the ships and goods in their convoy, without either diverting to other parts or occasions, or refusing or neglecting to fight in their defence, if they be assailed, or running away cowardly, and submitting the ships in their convoy to peril and hazard; or shall demand or exact any money or other reward from any merchant or master for convoying any ships or vessels entrusted to their care, or shall misuse the masters or mariners thereof; shall be condemned to make reparation of the damage to the merchants, owners, and others, as the court of admiralty shall adjudge, and also be punished criminally according to the quality of their offences, be it by pains of death, or other punishment, according as shall be adjudged fit by the court martial.
In the Royal Navy an Escort Vessel is any craft given the task of defending other ships from attack. In it's general form it refers to ships smaller than capital ships which accompany them to form a screen of some sort.

In the days of sail the Line of Battle Ship (later abbreviated to just "Battleship") was the premier weapon of war upon the high seas. The only threat was other Ships of the Line. Smaller ships, particularly Brigs, Sloops and Frigates, accompanied the Fleet, not to defend it, but to serve as Scouts and to pursue enemy ships too fast for the Line of Battle Ships to catch.

When not assigned to a Squadron or Fleet these smaller ships usually served one of two other purposes: Cruising - the hunting of enemy warships and merchantmen, and Convoy Escort - shepherding groups of merchantmen through potentially hostile waters.

When Nelson took his Fleet to Trafalgar he only took two Frigates with him, though he desperately needed all the scouts he could get to find the French/Spanish Fleet. The majority of his light forces had to remain in the Mediterranean to safeguard merchantmen from Privateers and Barbary Pirates eager to take advantage of the absence of the Fleet.

The nature of the Escort changed with industrialisation, the advent of the Torpedo, Mine and Submarine meant the Battleship was now vulnerable to lesser vessels and needed a screen of Minesweepers and Torpedo Boat Destroyers. Although these could be assigned to Convoy Escort duties it became apparent during WWI that a different type of ship was needed. The Fleet Minesweeper and Destroyer were designed for speed to keep up with the Fleet, they were poorly adapted for slow convoys over long distances.

WWI saw the construction of the first dedicated Convoy Escorts, classed as Sloops. But in a peace time Navy a dedicated Convoy Escort was too expensive to maintain and the Sloop evolved back into a fast, heavy armed Fleet Escort. By the advent of WWII the Royal Navy found itself yet again unprepared for the needs of defending merchantmen. The Corvette was born, a class of ship new to the RN and borrowed from the French classification of a an armed blockade runner.

The corvette was only a stop gap, a larger platform was needed for weapons and sensors and the disused class of Frigate was revitalised for the new breed of larger corvettes.

Post WWII the RN's main role in NATO was to protect reinforcements which would be rushed over from the USA so the need for Escort Vessels remained very much alive. WWII has seen the division of specialist Anti-Submarine and Anti-Aircraft Frigates and this continued post war, but gradually the Destroyer became associated with Anti-Aircraft and the Frigate with Anti-Submarine.

Battleships, Cruisers and Aircraft Carriers were seen as superfluous to the RN's role, and gradually were faded out, with the exception of the Escort Carriers which had proved their worth in both wars. For political reasons these were classed as "Through Deck Cruisers."

HMS Albermarle, a 28 gun Frigate captured from the French, pictured here in a 1905 edition of the Illustrated London News under the command of Captain Nelson escorting a convoy.

Japanese Print of the Battle of  Tsushima in 1905, small, cheap, torpedo armed boats attacked and destroyed a Russian Battlefleet.

An Atlantic convoy in 1917, at the start of the war convoys were not employed, despite their success in the Napoleonic Wars, it was felt a convoy would make it easy for commerce raiders to find targets. Ironically it was the French who requested the RN re-introduce them to preserve their strategic shipments of coal from the UK.

A chronic lack of suitable Escort Vessels at the start of WWII led to arming merchant ships as escorts. HMS Jervis Bay was a converted liner which was the only defense for convoy HX84, she was sunk defending it against the Pocket Battleship Scheer.