A holocron is a source of Jedi knowledge and teaching. They appear as palm-sized cubes of glowing crystal, etched with designs. Along with the Force, they use holograms to become interactive learning devices. Typically, they can only be activated by a Jedi. If activated, the user can seem to have a conversation with the Jedi whose teachings infuse the holocron.
Before the developers decided it should not be so, in Star Wars Galaxies, the player was required to collect Red and Blue holocrons (or Jedi and Sith holocrons) to become a Jedi. The Red holocron looked like a pyramid and the Blue one looked like a cube. Now, all the holocrons are useless. Even farther back, the player had to obtain many holocrons, but the game was changed because players were advancing to Jedi status much too slowly and farming the holocrons in massive amounts. These old holocrons were changed into "Holocron Shards" to prevent players from abusing the pre-nerf holocrons.