Photon torpedo
Photon torpedoes are secondary missile weapons used by the starships of the United Federation of Planets. They do damage via the release of gamma rays from a matter-antimatter reaction.
Federation starships have used weapons similar to photon torpedoes as well. How much--if any--they are based on photon torpedo technology is unknown.
Variable Yields
Photon torpedoes have been observed to have vastly different yields. Many of them can be explained through deliberate "dialing down" of the yields to avoid friendly fire, such as in The Final Frontier. However, a popular theory is the antimatter injection theory. Since the release of the High Definition version of The Wrath of Khan, torpedoes have been confirmed to have their antimatter inserted in "real-time." As well, torpedo casings are noted in numerous episodes without their warhead.
Therefore, the differences between the tactical yield and maximum yield is a logarithmic function of time, with more time required to ready the torpedo for a greater yield. In The Wrath of Khan special measures are taken to preload torpedoes in anticipation of battle.
Conveyor Belts vs Single Shot
As witnessed in The Wrath of Khan, torpedoes are stored and ferried to their launchers by a complex system of pulleys and conveyor belts. Such a system would be highly prone to failure under battle stress, and it is common for enemies to target specific weapons systems and disable them. Due to the "leaky" nature of Star Trek's shields, a pinpoint strike against a ship even with its shields up could knock out the conveyor system.
Recent incarnations of Starfleet ships such as the Enterprise-E have more torpedo launchers. These additional torpedo launchers are nowhere near the size of the torpedo bays on Galaxy-class, Constitution-class, or other Starfleet vessels. Rather than assume greater firepower due to increased number of torpedo launchers, another more elegant explanation is the Federation attempting to do away with this conveyor system.
One example is the single-shot launcher on top of the observation lounge on the Enterprise-E. Either the launcher is single shot, because there is no room for the complex machinery required for the conveyor system, or there is a hard line and the torpedoes are transported to the launcher. Another possibility is a site-to-site transport. All of these approaches present serious problems of their own, such as greater power usage and complexity for no gain.
The most satisfying solution would be to not use antimatter torpedoes at all, a position the Federation seems unprepared to adopt.