Spiritual lifeforms. Astral being is a general term for beings found on planes of existence higher than the physical plane.
An astral being need not hail from the astral plane. It may be a being from any of the other higher planes.
Intelligence varies widly among the many forms of astral beings. The majority of astral beings exhibit low intelligence. Generally, the higher the plane a being comes from, the greater the intelligence.
Size, as a rule, has little to do with ability when it comes to astral beings. In fact some of the most powerful beings a projector will come across appear as minuscule creatures.
Spiritual icons such as angels and demons may be astral beings that contact human beings for one reason or another. If this were the case, it would seem that they, like humans, have an ability to visit other planes than the one of their residence.
Projectors will often find that many astral beings are completely unaware of other planes of existence or even of the projector himself.
Occasionally, a projector will stumble upon (possibly be guided to) a ceremony being conducted by higher level spiritual beings. These ceremonies usually mark a major step in the spiritual existence of the projector.
Spiritual beings should be approached in a very neutral and/or peaceful manner if at all. It is unclear as to whether or not they are actually capable of harming astral travelers, but I wouldn't want to be the one to find out.