Here's what The Making of Star Trek
has to say on the subject:
The Enterprise is equipped with a second type of offensive weapon, called photon torpedoes, which are energy pods of matter and anti-matter contained and held temporarily separated in a magno-photon force field. These can be used as torpedoes or depth charges, and can be set with electrochemical, proximity, and a variety of other fuses. Photon torpedoes can be fired directly at a target, laid out as a mine field, or scattered in an attacker's path as depth charges.
So, at least as originally conceived, photon torpedoes were physical ordnance analogous to naval torpedoes, mines or depth charges, depending on how they were used.