it is quite complicated but basically:
An ordinary photo doesn't appear 3d, this is primarily because the same image is being fed to both eyes. Also, if you tilt a photograph you cannot see round the objects in it.
A hologram is an image that appears 3d and contains objects that can be seen around. this is due to the fact that every particle in a holographic recording medium contains the same object from its own unique point of view. therefore, the each eye can pick up an individual image depending of which way you are looking at the hologram.
This means that if you were to smash your holographic plate into small pieces, you would still be able to project the full image onto a wall with a laser.
Holograms don't necesarilly refer to a projection of an object in a room(like the doctor in startrek voyager). The ordinary type of hologram is much like a photo, but 3D. The sort of hologram I am trying to create is of a small object such as a key. The equipment required usually involves a laser, an object, a sturdy table, mirrors, a recording medium (plate) and processing chemicals.
for more info:
http://holostar.cmaisonneuve.qc.ca/en/index.html