Astral Projection is essentially your soul leaving your body and entering the Astral realm. A lot of people believe that you do it every night when you dream. Astral projection is just the controlled method of doing this. When you manage to AP, then in theory you can go anywhere you like. You just think about where you want to be and you'll find yourself there. But it's not easy. I've been trying to do it on and off for about three years, i've come close, but never actually managed it entirely.
But i do know some techniques. This one is supposed to be one of the easiest:
You need to completly relax. Do this by focusing on each part of your body in turn. Make sure you focus on your breathing, making it steady and slow. Once you're relaxed, make sure you stay completly still. You may feel the need to cough or to scratch, you may become uncomfortable and want to move. DON'T. You'll have to start again.
Essentially, what you are trying to do is enter a state where your body is asleep, but your mind is awake. So think about anything and everything. Eventually, you'll begin to lose awareness of your body. This is what you want to happen. When it does eventually happen, think about how nice it would be to leave your body. Feel yourself getting lighter and lighter. On every in breath, you are floating higher and further away from your body.
You may start to get these weird tingles through your body, and you may hear strange noises, but carry on it's nothing to worry about. Eventually, you'll kind of feel yourself seperate from your body, it's weird, (and the reason i've never got any further cos it freaks me out) It feels like your sitting up and just floating above your bed. Thats as far as my experience goes, but most things i have read on the subject, advise you to move away from your body before opening your eyes, as seeing your physical form will almost certainly jolt you back. From here on, you can do what you like. As long as you don't want to spy on people in the physical world, or enter anyone's home without permission. If you visit the Astral realm, look out for negative entities. Apparently you can tell if their bad by looking in their eyes.
I'm sure everyone else will have different thoeries and methods, but thats sort of what it's about, there are hundreds of ways to AP, you just have to find the way which works for you.
Apologies for the long post.