Photon Missiles are powerful and dangerous space age weaponry.
Photon Missiles are missiles that hold matter and anti-matter, or sometimes just anti-matter, which on impact mix to cause the anihilation effect which releases a devastating burst of energy. An Anti-matter bomb the payload of the Hiroshima Bomb would cause an explosion that could be seen from space. The reason for the name Photon Missile is because the explosion is accompanied by a large burst of light. Anti-Matter explosions also release Delta Radiation.
Photon Missile generally just look like high tech missiles considering only Space Age Civilizations can create them.