Everyone experiences astral travel differently. Much of it is determined by how your mind can assimilate and process the information it receives. As far as the planes, levels, spirits, etc. They will vary with experience and each individual as well, and people of different traditions will tell you different things.
To my experience, the astral is sort of a place between realms and those other realms that exist are nearly infinite. The spirits that dwell within may also be travelers from the various realms, concentrations of energy that have become self aware or otherwise programmed through any manner of process (thought forms, servitors, etc) gods, angels, demons, spirit animals, lost souls, etc. etc. If it has been encountered anywhere, it can be encountered in the astral. But the astral is also so vast that generally speaking if you're not looking for it or waving around a big beacon of energy you're probably not going to run into something you don't want to. That being said, it's also very easy to just make yourself small and invisible in the astral...
Bottom line, the astral is very difficult to define in specific terms or rules as it is the convergence of all things spiritual. And all things spiritual tend to be adept and avoiding true consistency or specific rules or else they would already be incorporated into the laws of physics.