The depth charge is an anti-submarine weapon intended to defeat its target by the shock of exploding near it. Most use explosives and a fuse set to go off at a pre-determined depth. Some have been designed to use nuclear warheads. Depth charges can be deployed by both ships and aircraft.

History
The first delivery mechanism was to simply roll the "ashcans" off racks at the stern of the attacking vessel. Originally depth charges were simply placed at the top of a ramp and allowed to let roll. Improved racks, which could hold several depth charges and release them remotely with a trigger, were developed towards the end of the First World War. These racks remained in use throughout World War II, because they were simple and easy to reload.
Some Royal Navy trawlers used for anti-submarine work during 1917-1918 had a thrower on the forecastle for a single depth charge, but there do not seem to be any records of it being used in action. Specialized depth charge projectors were developed to generate a wider dispersal pattern when used in conjunction with rack-deployed charges. The first of these projectors, produced in 1918, were called Y-guns in reference to their basic shape. Mounted on the centerline of the ship with the arms of the "Y" pointing towards the sides of the ship, a depth charge was cradled on a shuttle inserted into each arm. An explosive propellant charge was detonated in the vertical column of the Y-gun to propel a depth charge about 150 feet (50 meters) over each side of the ship. The main disadvantage of the Y-gun is that it must be mounted on the centerline of a ship's deck, which may otherwise be occupied by superstructure, masts, or gun turrets.
The K-gun, made standard in 1942, replaced the Y-gun as the primary depth charge projector. K-guns could be mounted on the periphery of a ship's deck, thus freeing up valuable centerline space. The K-guns were often used together with stern racks to create patterns of six to ten charges. In all cases, the attacking ship needed to be moving above a certain speed or it would be damaged by the force of its own weapons.
Depth-charges can also be dropped from an attacking aircraft against submarines. At the start of World War II, Britain's aerial anti-submarine weapon was the 100 lb anti-submarine bomb. This weapon was too light and ultimately, a failure. Indeed, on September 5, 1939, a Royal Air Force Avro Anson of 233 squadron was destroyed when its own A/S bomb skipped off the surface of the water and detonated under the aircraft. To remedy the failure of this weapon, the Royal Navy's 450 lb Mark VII depth charge was modified for aerial use by the addition of a streamlined nose fairing and stabilising fins on the tail. Later depth charges would be developed specifically for aerial use. Such weapons still have utility today and are in limited use, particularly for shallow-water situations where a homing torpedo may not be suitable. Depth charges are especially useful for 'flushing the prey' in the event a diesel submarine is lying on the bottom or otherwise hiding with all machinery shut down. Homing torpedoes can be used for the same purpose, but the cost is prohibitive and aircraft and shipboard inventories limited. An example of such a weapon is the BAE Systems Mark 11, deployed by the British Fleet Air Arm.

Delivery Mechanisms
The effective use of depth charges required the combined resources and skills of many individuals during an attack. Sonar, helm, depth charge crews and the movement of other ships had to be carefully coordinated. Aircraft depth charge tactics depended upon location of the sub during the day and at night, then quickly attacking once it had been located, as the sub would normally crash-dive to escape attack.
As the Battle of the Atlantic wore on, British and Commonwealth forces became particularly adept at depth charge tactics, and formed some of the first destroyer hunter-killer groups to actively seek out and destroy German U-boats.
The shortcoming of the depth charge as deployed by surface ships was not the weapon itself, but how it was delivered. An attacking vessel would usually detect a submerged contact using its Asdic (or in American parlance, Sonar). However, to drop its depth charges it had to pass over the contact to drop them over the stern. As such, Asdic contact would be lost immediately prior to attack, thus rendering the hunter blind at the crucial moment. A skillful submarine commander therefore had an opportunity to take successful avoiding action. This situation would be remedied by the adoption of the ahead-throwing weapon, allowing contacts to be engaged at a stand-off distance while still in sonar contact.

Later Developments

Naval mine
Nuclear Depth Bomb