Holocrons are small devices that store information, which are presented by a interactive holographic representation of a Jedi or Sith who created it, the "gatekeeper". The Gatekeeper relays information, stories, lessons, etc. Holocrons usually require the user to be Force sensitive in order to access the infomration. Jedi Holocrons are typically cube-shaped, while Sith Holocrons are usually pyramids. The AOTC Visual Dictionary also features something called a Great Holocron, which is multi-faceted and contains a great deal of knowledge. [image=http://www.3k.org/jedi/images/holocron2.gif] [image=http://members.aol.com/drkfathr1/Sithholocron.gif] [image=http://www.jediacademy.com/oracle/holocron.gif]
Luke used Jedi holocrons. The first one we see him using Leia took from Palpatine in DE, though Palpatine had taken it from a Jedi Master his forces had captured so Leia basically reclaimed it for the Jedi. Jedi Holocrons like the one Luke used are cube shaped just like the first image 2ndQuest posted. The second image is a Sith Holocron. The third image appears first in the AotC Visual dictionary and if memory serves from the few times I skimmed the book in the bookstore, it's a more modern design of the Jedi Holocron.
I've seen a real one! it was so cool. it was the sith holocron that they used in the AOTC visual dictionary it had aurbish writing on it, and it said something really cool (however I forgot what it did say...) anyway very cool little prop! Forcebewitya!