A hologram is a recording of an interference pattern made by the interaction of two beams of light. In this explanation, the recording is made in the light-sensitive emulsion of a photographic film.
If two stones are dropped into water, waves radiate from the points of impact. Where the two sets of waves meet, the waves appear to pass through each other; however, where a crest of one wave coincides with a crest of the other wave, an extra high crest will form. Similarly, two troughs coinciding will form an extra low trough. A crest and a trough meeting will cancel each other out. Since light travels in waves, two light beams crossing will have similar crests, troughs and level patches, forming light and dark fringes. If a suitable photographic film is inserted where the beams coincide, it can record this fringe pattern.
In holography, one beam (the reference beam) comes directly from the laser, while the other (the object beam) comes from the same laser but impinges on the object, and is distorted by it, before striking the photographic film. What is recorded on the film is the interference pattern produced by the two beams. After development, if the hologram is illuminated by a beam of light from the direction of the reference beam, the object beam is recreated, and the object "appears."
Laser light is used in holography because it has coherence. This means that the waves of light coming from the laser are not only of the same wavelength, but that all the waves are "in step", like soldiers marching together. White light, from the sun or a light bulb, is made up of many different wavelengths, so it is like a group of people walking along, some taking short strides, some longer ones, all out of step, and going in various directions.
