What is a Hologram ?

A hologram is a record of the interaction of two beams of coherent light, in the form of microscopic pattern of interference fringes. It is a photographic registration of the interference pattern formed by two laser beams of coherent light. One beam goes straight from the light source and the other is scattered from a physical object. The photographic film or plate is exposed by two laser beams and is processed in such way that when illuminated appropriately a three-dimensional image is produced.

.....this is just a spark and i would like you people to contribute your knowledge about it to turn this spark into a big bonfire...

I remember seeing a demonstration with holographs. After a holographic slide was produced from the interference of a divided lasar beam, the demonstrator took scissors and cut out a pice from the slide and the whole image was still prodcued within the fractional aspect of the slide.