There are many definitions of astral projection however I will explain it the way I see it.
First you need to understand there are different planes of existence, we live in the physical plane. The astral plane is the spiritual plane in which our higher selves exist. I believe that our physical bodies are directly rooted in the physical plane where as our spirit or soul etc. is tethered to the astral plane. So when we astral project we our projecting our consciousness from the physical plane into the astral/spiritual plane. Within the astral plane the confines of physicality no longer is take place, our perception is in a state of lucidity, which is not to be confused with lucid dreaming as that in another subject altogether.
The state of the astral plain itself or the experience of the individual is directly linked to the consciousness of the projector and his or her spiritual being. No one can accurately describe or tell you what you may experience as each experience is unique to everyone. There have been documented similarities but if I were to astral project and then describe my experience, what I saw, who i saw, what I did etc. it would not be the same for you.
Astral projection and the astral plane is not to be confused with or mistaken as the Etheric plane or Etheric Projection. as the Ethereal plane is a hyperplane of the physical plane that we live in now. As other physical planes constituting solid, liquid, and gas, where as the ethereal plane is described as a web or something else that acts as a threshold or barrier that holds the other three into place.
As far as methods of astral travel, many people first learn the basics of magic, they study, learn and practice them. Especially meditation and visualization as those seem to be the two most common methods of transport into the astral plane i.e. an astral vehicle of sorts. Other methods exist such as drug induced astral projection, and so on.
I hope this helps in your understanding of astral projection.
Blessings and Peace be with you,