Principle of a Hologram
Holography is based on the principle of interference. A hologram captures the interference pattern between two or more beams of coherent light (i.e. laser light). The hologram captures light as it interests the whole area of the film, hence being described as a 'window with memory'.
How Hologram is made
Holograms are made by using lasers. The reason that they use lasers instead of regular (white) light is that laser light is "coherent." What that means is that all of the light waves in a laser beam are the same size, and vibrate at exactly the same time. Because they're all the same size, all of the waves will bend the same way. (If you've ever shined a white light through a prism, you know that the different colors of light will bend differently - that's because all the different colors have waves that are different sizes. If you shine a laser through a prism, the entire beam bends together, since all of its waves are the same size.)
Holograms are made by using a single laser beam. The beam is then split into two beams by a special lens. That way, you get two laser beams that are exactly the same. One of those beams is the "reference beam" and is shone directly onto the film. (The film is basically the same stuff as a regular photo film.) The second beam is reflected off of the object that you want to make a hologram of.
When the two laser beams intersect, they create what’s called an interference pattern. This is the pattern that the two sets of waves make when they overlap. (To picture this, you can imagine if you dropped two pebbles into a puddle. The pebbles make waves that go outwards, and when the two sets of waves run into each other, they form a pattern.) That pattern is what’s recorded in the film. Then when the film is developed, you can see the whole image.
The interesting thing about holograms is that when the image is recorded onto the film, the entire image is recorded onto the whole piece of film, so if you cut the film in half, you can still see the whole picture. It’s like when you look out your window. If you only look through half of the window, you can still see the whole picture outside.
There are actually a lot of different setups that can be used to make holograms, but they all follow the same basic idea that this one does.