When we astral project we enter an "astral body," which is separate from the physical body and allows us to travel outside of it.
The Astral Realm, or "Astral Planes," is a world that is much like ours, but vibrates on different frequencies. Having levels that hold entities and beings that vibrate on a certain vibration.
Lower vibrational beings live on the lower planes; these beings are negative. Positive entities live on the higher planes and vibrate much faster than lower entities.
We give out a vibration depending on our mood; if we are depressed, we give out a low frequency that will allow negative entities to see us, and may be drawn towards us to feed off that energy. If we are happy and confident, then we give off a higher frequency. So lower frequencies equal lower planes, and higher frequencies equal higher planes.
Anything that you can imagine is possible when you astral project. If you wanted to visit a different planet, you could. If you wanted to fly over cities, you could.
People experience astral projection either spontaneously by illness, sleep, surgical operations, sleep paralysis, or it is induced by practicing meditation.
People sometimes mistake lucid dreaming (Being aware that you are in a dream and being able to control what's in it) for astral projection. The difference is that when you astral project, you are actually going to these places. When you lucid dream, you're obviously still dreaming.
People astral project for many different reasons: for spiritual beliefs, to gain knowledge and interact with the other side, or some people may just be curious.
We astral project every night when we sleep, we are just not consciously aware of it.