Torpedo

A Torpedo is a self-propelled, self-guided weapon commonly found in the arsenals of many starships.

Torpedoes are used in space combat to compliment, or as an alternative, to energy weapons such as phasers as disruptors. Most torpedoes are significantly more destructive and less precise than directed energy weapons so are only used when one desires the opponent destroyed or severely damaged.

History and Types
Torpedoes used by Humans date back to usage on navel vessels. The first human spatial torpedoes were introduced in the 22nd century (Enterprise episode: Fight or Flight). In the same time frame the Vulcans already used photonic weapons and the Klingons armed their ships with Photon torpedoes (Enterprise episodes: Awakening and Sleeping Dogs). Earth was quick to catch up and soon developed their own photonic weapons which were brought into service in 2153 when the Enterprise NX-01 was retrofitted to launch them for her mission into the Delphic Expanse (Enterprise episode: The Expanse). By the 23rd century the Federation Starfleet had advanced there weaponry to photon torpedoes, though the basic case design and technology of matter/antimatter warhead wasn't all that different from the earlier photonic torpedoes.

The Federation and many other races use photon torpedoes and they remained the most commonly used torpedo on Federation starships well into the 24th century. Despite a few small wars the period from the 23rd to 24th century was a peaceful one for the Federation so weapons development was not a high priority. This changed with first contact with the Borg. Realising they were severely outmatched Starfleet quickly began to develop new weapons, starships and tactics (TNG episode: The Best of Both Worlds). One weapon to come out of this effort was the Quantum torpedo which was first used in the Defiant class, which was also produced as part of the anti-Borg efforts (DS9 episode: The Search).

Whilst Photon torpedoes are quite a common weapon not all races use them, some prefer plasma torpedoes.

Following Quantum torpedoes the next stage in torpedo design would seem to be the Transphasic torpedo, a very powerful weapon brought back in time from the early 25th century by Admiral Janeway. These torpedoes were designed specifically for use against the Borg and a single transphasic torpedo is capable of destroying a Borg Cube (Voyager episode: Endgame). Whether or not Starfleet will make use of the weapons in the near future is unclear, without a severe threat to necessitate their use the political effect of using such advanced weaponry might not be worth having the technology widely available. Exactly what the temporal prime directive has to say about use of future technologies is also unclear.