If I may be forgiven for quoting Wikipedia:
Escort Carrier
Some snippets: In the Battle of the Atlantic, escort carriers were used to protect convoys against U-boats. Initially escort carriers accompanied the merchant ships and helped to fend off attacks from aircraft and submarines. As numbers increased later in the war, escort carriers also formed part of hunter-killer groups that sought out submarines instead of being attached to a particular convoy...
The Royal Navy had recognized a need for carriers to defend its trade routes in the 1930s. While designs had been prepared for "trade protection carriers" and five suitable liners identified for conversion, nothing further was done mostly because there were insufficient aircraft for even the fleet carriers under construction at the time. However, by 1940 the need had become urgent and HMS Audacity was converted from the captured German merchant ship MV Hannover and commissioned in July 1941. For defense from German aircraft, convoys were supplied first with fighter catapult ships and CAM ships that could carry a single (disposable) fighter. In the interim, before escort carriers could be supplied, they also brought in merchant aircraft carriers that could operate four aircraft...
In all, 130 Allied escort carriers were launched or converted during the war. Of these, six were British conversions of merchant ships: HMS Audacity, Nairana, Campania, Activity, Pretoria Castle and Vindex. The remaining escort carriers were U.S.-built. Like the British, the first U.S. escort carriers were converted merchant vessels (or in the Sangamonclass, converted military oilers). The Bogue-class carriers were based on the hull of the Type C3 cargo ship. The last 69 escort carriers of the Casablanca and Commencement Bay classes were purpose-designed and purpose-built carriers drawing on the experience gained with the previous classes...
The Attacker class is an example - from Wikipedia again:
The United States Navy had acquired 22 C3 cargo ships shortly after the Attack on Pearl Harbor to be converted into the Bogueclass. With an increasing need for convoy escorts in the North Atlantic eleven of these were transferred to the Royal Navy, and reclassified as Attacker class, under the terms of the Lend-Lease program.
The ships were originally intended to serve as convoy escort carriers, equipped with both anti-submarine and fighter aircraft, and transport carriers, transferring new and replacement aircraft to forward bases. During successful use during the amphibious invasion of North Africa to cover advancing ground units until land airbases were secured, several ships were refit as strike carriers, equipped with just fighter aircraft. When used as convoy escorts, the ships' aircraft were successful in deterring German submarines from attacking Allied convoys, with a number of German submarines and aircraft destroyed or damaged by the aircraft...
All the escort carriers had the capacity for up to 20 anti-submarine or fighter aircraft, which could be a mixture of the British Hawker Sea Hurricane, Supermarine Seafire, and Fairey Swordfish, and the American Grumman Wildcat, Vought F4U Corsair and Grumman Avenger. The exact composition of the embarked squadrons depended upon the mission. Some squadrons were composite squadrons for convoy defence, and would be equipped with anti-submarine and fighter aircraft, while other squadrons working in a strike-carrier role would only be equipped with fighter aircraft...
Escort carriers were designed to accompany other ships, forming the escort for convoys. The anti-submarine aircraft employed were initially Fairey Swordfish and later Grumman Avengers, which could be armed with torpedoes, depth charges, 250-pound (110 kg) bombs, or the RP-3 rocket projectile. As well as carrying out their own attacks on U-boats, these aircraft identified target locations for the convoy's escorts to attack. Typically anti-submarine patrols would be flown between dawn and dusk. One aircraft would fly about 10 miles (16 km) ahead of the convoy, while another patrolled astern. Patrols would last between two and three hours, using both radar and visual observation in their search for U-boats. By 1944, it was usual to have two escort carriers working as a pair on convoy escort. Experience had shown it was best to have two composite squadrons. One squadron included fighters and the by then obsolete Fairey Swordfish equipped with air-to-surface vessel (ASV) radar for night patrols. The other squadron would be equipped with fighters and the Grumman Avenger for long-range day patrols, as they could not be fitted with the ASV radar.
The Fleet Air Arm squadrons flying off the Attacker class escort carriers did have some successes of their own. The first of six confirmed U-boats destroyed by aircraft flying off Attacker class ships was on 10 February 1944, when two Fairey Swordfish from the 842 Naval Air Squadron on board Fencer sank U-666 west of Iceland. On 4 March, while on Arctic convoy patrol, Fairey Swordfish from 816 Naval Air Squadron on board Chaser so severely damaged U-472 with a salvo of RP-3 rockets that she could not submerge and was sunk by HMS Onslaught* For the rest of the day Chaser's Fairey Swordfish kept the U-boats at bay by identifying their locations to her escorts. They also damaged two other U-boats themselves. The U-366 was sunk by RP-3 rockets fired from a Fairey Swordfish on 5 March, and the U-973 on 6 March. Three other U-boats sighted managed to evade an attack in foggy conditions. Operating from Fencer, 842 Squadron sank their second submarine, U-277, on 1 May, and sank U-959 and U-674 on 2 May 1944.
The carriers' aircraft could also claim some success against the Luftwaffe's long-range bombers. On 1 December 1943, two Grumman Wildcats from 842 Naval Air Squadron on board Fencer shot down a Focke-Wulf Fw 200 that was spying on Convoy OS 60. The next confirmed air-to-air success came on 24 February 1944, when four Grumman Wildcats from 881 Naval Air Squadron on board Pursuer were scrambled after the ship's radar had identified at least three aircraft approaching. The approaching bombers consisted of a mixed force of seven Focke-Wulf Fw 200 and Heinkel He 177s carrying glider bombs. One Fw 200 and one He 177 were shot down by Grumman Wildcats. The rest of the Germans kept their distance due to the combined efforts of the fighters and the ships' anti-aircraft fire. Off Cape Finisterre in March 1944, Grumman Wildcat fighters from Pursuer shot down a Heinkel He 177 and a Focke-Wulf Fw 200, and damaged a Fw 200.
In August 1944, the Arctic convoys had started again, the first one being escorted by Striker and HMSVindex, a British-built escort carrier. On board Striker was 824 Naval Air Squadron with twelve Fairey Swordfish IIs, ten Grumman Wildcat Vs, and two spares. The Grumman Wildcats shot down a Blohm & Voss BV 138 on 22 August. For Operation Neptune from 5 June 1944 to the middle of the month, five all-fighter escort carriers, including Fencer, provided air cover to protect the anti-submarine groups on the flanks of the Normandy invasion fleet...
*It was common for a U boat kill to be shared between an aircraft and the surface escorts. This might be the
aircraft spotting a surfaced U-boat, or being cued by HF/DF. If a U boat sighted an aircraft it would usually crash dive, but its position was now known to the convoy and escorts. Swordfish (etc) might well be go in with rockets, which would not directly sink a U-boat most times, but could hole the pressure hull and trap the submarine on the surface: for surface units to close in, and have the U-boat either make a doomed last stand or get the crew off and scuttle.
The convoys and support groups also worked with long range land based types such as Sunderlands from RAF Coastal Command, but the heavy aircraft carried weapons able to achieve an outright kill.