The proton torpedo was a type of ordnance used throughout the galaxy. Its most notable use was during the Battle of Yavin, when Luke Skywalker destroyed the Death Star I by launching a pair of proton torpedoes into a thermal exhaust port.
Proton torpedoes came in a wide array of sizes and yields; from massively powerful capital ship weapons designed for orbital bombardment, to small but nimble fighter-grade weapons like those Luke Skywalker employed against the first Death Star. A number of torpedo designs were shielded to protect them from being shot down either by skilled pilots or the target's defensive flak guns.
Small starfighters like the Rebel Alliance T-65 X-wing and Y-wing fighters often had their laser weaponry complemented by ordnance launchers capable of firing proton torpedoes. The energized torpedoes carried a devastatingly powerful blast, and could be fired at much greater ranges than the standard laser cannon. The torpedoes were exceptionally maneuverable, able to make a 90 degree turn within a turning circle of a meter, enabling them to be used against starfighters, such as when Corran Horn in his X-Wing used them against Erisi Dlarit's TIE Interceptor.
During the Rebel attack run on the Death Star I, Alliance fighters had to use high-speed proton torpedoes on the thermal exhaust port that protected the Death Star's sole vulnerable point. Since the port was ray-shielded, standard laser weaponry would have been ineffective. Proton torpedoes were also used during the Battle of Naboo by Naboo N-1 Starfighter pilots. Anakin Skywalker accidentally crash landed in the Droid Control Ship and destroyed the control ship using proton torpedoes.
In the X-wing series of computer games, proton torpedoes were intended to be used against most targets except snubfighters, as they were slower than concussion missiles but more damaging. However, this contradicts information in many canon sources which show that both proton torpedoes and concussion missiles are just general types of weapons with various yields. Because of that, this fact should be dismissed as a mere game mechanic.
In Star Wars: TIE Fighter, at the hangar screen where the player has the option of chancing the proton torpedo is depicted as a stubby blue missile, which contradicts canon showing that the torpedo is cone-shaped.
According to the Star Wars Sourcebook: A New Hope for the roleplaying game, proton torpedoes could do more damage (but not significantly more) than concussion missiles and cost less per warhead. However, complete particle shielding could deflect proton torpedoes successfully.
Although Jedi Trial states that proton torpedoes were a new weapon introduced during the Clone Wars, the N-1 starfighter is known to have been equipped with the weapons during the Battle of Naboo. Additionally, in Star Wars: Knights of the Old Republic II: The Sith Lords, Atton Rand makes a passing reference to proton torpedoes, indicating that the weapon may actually have originated far prior to the conflict. And if the prologue for the game is played, a proton torpedo can be found stuck in the hull of the Ebon Hawk and the explosive charge is found intact, and can be used as an explosive. It is also a rare case of a Proton Torpedo not exploding on contact.
In Star Wars Battlefront II, TIE fighters were equipped with proton torpedoes. However, TIE fighters never had these in the storyline.
The exact nature of Proton Torpedo warheads are not fully understood, since few sources talk about the actual nature of the warhead. However two separate authors have in the LOTF series stated that they use a baradium main charge, detonated by a Proton Detonator. Some sources state that the blast is focused into a tight cone to maximize effectiveness and minimize collateral damage and energy leakage away from the target. However, most sources particularly those detailing the effects of a hit on a starfighter, have these weapons producing a more uniform detonation. This is particularly demonstrated when Wedge Antilles used one such weapon to destroy a concrete tube, the Torpedo passed through the tube and detonated ten meters away, yet still produced a fireball sufficient to engulf the tube.