(covers information from Warship Voyager)
Photonic Torpedo (or Pu'DaH dak cha in Klingon) are a tactical Matter/Anti-Matter light energy based projectile weapon that is used as a Primary Weapons System used on most War and Starships within the known Milky Way Galaxy.
The torpedo itself is a Solidified energy pod of pure photon particle energy, encased within the pod is a smaller Nanoparticle of Ultrareactive Anti-Matter consisting of pure Anti-Deuterium in order to create the fission explosion. The entire Pod is also encased in a Elliptical Phased Nadion particle plasma energy based cylinder called a Phaser Tube. Before firing from its launcher.
The torpedo's Matter/Anti-Matter charge is held temporarily separated by a internal Magno-Photonic Force Field and can be activated with Electrochemical based detonators. When launched the torpedo creates a trail of Phased plasma exhaust which creates a small "Comet" effect away from the firing ship.
The torpedo can fire through a ships activated shields with some of the same mechanics as the pulse phaser, however its system is much easier to enable by simply making the torpedoes energy frequency opposite from the Shields frequency modulation which allows the projectile to easily pass through the ships shields.