Holocrons are a crystal lattice (often in cube form) in which people of old could insert phenomenal quantities of data. In the era depicted in the Star Wars films, the technology to make them is nearly lost. But the few surviving holocrons contain information on the Jedi arts, blueprints of ancient ships and histories of the universe. Jedi Holocrons are a sub-class of holocrons which are made by Jedi, and contain valuable information about Jedi techniques, training methods, and lightsaber creation. Usually, only Force-attuned users can access a Jedi Holocron, while other holocrons may be accessed by ordinary individuals, usually by manipulating the facets of the holocron (as shown in the novels Darth Maul: Shadow Hunter and in Cloak of Deception).
There exist Sith holocrons which are often in pyramidal form. The Great Holocron, a 12-sided holocron stored in the library of the Jedi Temple, is said to contain the greatest concentration of collected Jedi knowledge. It was also designed to be extremely easy to use for those sensitive to the Force, especially Padawans
Holocrons can also be used as a form of recording device for personal thoughts. In the game Knights of the Old Republic II: The Sith Lords, a character following the dark side of the Force could discover a Sith Holocron on Korriban containing a recording of Bastila Shan, the apprentice to the Sith Lord Darth Revan, detailing her decision to follow him into the Unknown Regions.
A few depictions of holocrons seem to indicate that they are sufficiently sophisticated to support an intelligence inhabiting the holocron; in Light side Jedi holocrons, these entities' personalities are patterned after a Jedi Master who added a great deal of information to the holocron, and are referred to as "Gatekeepers" (as in, "The Gatekeeper of the holocron will refuse to divulge information to non-Force sensitives.") Sith holocrons are rarely seen. Two incidents point to Sith holocrons being imbued with a malevolent intelligence capable of directly influencing their users, unlike the Jedi Holocrons, which have no such documented capabilities. In the game Knights of the Old Republic II: The Sith Lords, the Jedi Exile is caught in a duel with Atris after her fall to the dark side and learns that the Sith Holocrons surrounding her actually speak to her. In the Tales of the Jedi collection, Tales of the Jedi: The Sith War, Exar Kun shatters a Sith holocron in front of a crowd of Jedi students who had followed him to his strongholds on Yavin 4. The fragments of the holocron hit the students, and then sink into their flesh, brainwashing them into becoming loyal Sith. Kun then is capable of ordering them to betray, hunt down, and kill their masters.
Holocrons made their first appearance in the 1992 Dark Horse Comics series Dark Empire. The resurrected Emperor Palpatine gives one to Leia Organa Solo; the holocron possesses the repository of knowledge of an ancient Jedi Master named Boda Baas, with whom she could interact via a hologram of the Jedi emitted from the holocron.
In the game Star Wars Galactic Battlegrounds, holocrons can be found lying around in the battlefield. Using a Jedi or Sith to transport the holocron to a Jedi or Sith temple can give a continuous source of nova crystals. Capturing five holocrons is one way to win the Standard Game.
Internally, however, Sith holocrons work in much the same way as do a Jedi holocron. The black crystal of the Sith holocron is made up of intricate lattices and vertices, capable of storing massive amounts of information, as well as the holocron's Sith gatekeeper. The holocron can capture the appearance and cognitive networks of the holocron's owner, and transform this into a three-dimensional hologram. This gatekeeper acts as a guide through the holocron's store of information, as a search and recovery engine with a more personal touch. These gatekeepers could relate stories from the holocron's archives to events in the life of the one who activated the holocron.
Jedi gatekeepers could sense the ability and level of a Jedi, and offer or restrict information accordingly — so that Padawans were not shown information too advanced and dangerous for their basic stage. However, Sith holocrons usually allow the activator unlimited access to its entire dark side repository. The gatekeeper would draw the viewer in with tales of power, and could even turn some Force-users to the dark side. The Jedi historian Atris discovers the Holocron's pull as she collects them; they slowly turn her to the dark side. Only one known Sith holocron exists at this point, having been captured by Odan-Urr during the Great Hyperspace War; 40 years later, Atris keeps scores of them in the Telosian Jedi Academy. The holocron captures by Odan-Urr is said to contain information dating back 100,000 years.
As of the prequel films' timeline, holocrons are locked away in the Jedi Archives to all but the most senior Jedi — in fact, most of the Jedi in the Temple do not even know Sith holocrons reside in the building. The holocrons are kept in case the Jedi would ever face the threat of the Sith again, whereupon Jedi Masters could consult the holocrons for ideas on how to defeat them. In Legacy of the Jedi, padawan Lorian Nod steals a Sith holocron from the Archives, though the theft ends in his expulsion from the Jedi Order. His friend and fellow padawan Count Dooku steals at least one Sith holocron from the Temple during the First Battle of Coruscant. In Dark Lord: The Rise of Darth Vader, Palpatine reveals that the remaining Sith holocrons in the Temple are forgeries, and that he had inherited several real ones from his master, Darth Plagueis.
In Star Wars Republic 63: Striking from the Shadows, Dooku presents a Sith holocron created by Darth Andeddu, an ancient Sith Lord. This holocron is recovered during a competition between Quinlan Vos and Tol Skorr, two of Dooku's Dark Jedi apprentices. It contains much dark side knowledge as well as a red lightsaber crystal embedded in its core, used to power Darth Andeddu's own Sith lightsaber. Dooku keeps the holocron itself, while giving the red crystal to Vos to use in his lightsaber.
In Star Wars Galaxies the Massive Multiplayer Online Role-Playing Game or MMORPG put out by Lucas Arts and Sony Online Entertainment, Holocrons were a key part of unlocking a Jedi character slot. A player could loot holocrons and use them as guidance to force sensitivity. Since the game has been revamped, however, holocrons are used primarily in the "collections" system and for decoration.