I can only refer to Castanedas books. Contact me if you wish to add something.

In the first books, they are descibed as abstract forces or spirits, which can be used to some extent by a sorcerer. This part of the description does not change in the later books, but don Juan later recommends Castaneda to have little to do with them, while in the first books he is convincing Castaneda that he should try and 'get one'. In the middle books it is said that a sorcerer can 'have' allies, and that a sorcerer keeps them in a calebass in his belt. In 'Art of Dreaming' the description of the allies change a great deal.

According to Castaneda, and according to many other sources as well, the universe is split into several 'layers', like the skins of an onion. There are ten such layers (in some other systems the number of paralell universes may differ. I believe that the Siberian shamans have nine such worlds, divided into underworlds and heavens, another common figure is seven). The 'Inorganic beings' come from another such layer than ours. They are (still according to Castaneda) only interested in energy, and as sorcerers go they are concerned with saving energy and thus become very interesting to the allies. The allies tries to make bonds with the human practitioners, and invites the practitioner to come and 'live' in them.

There are, however, already in the early books some practices discribed which are associated with 'hunting power', and where the use of allies come into practical use.

In the ordinary world the inorganic being manifests themselves in different (normal) shapes and happenings, sometimes as people. According to don Juan, one can learn a great deal by observing them.

The inorganic beings are described as very dangerous in all the books.

In 'Art of Dreaming', the inorganic beings are described as stationary, like trees. What is seen of them 'here' by practitioners and others are mere projections, or scouts. The inorganic beings send 'scouts' into our dreams, and try to get dreamers to follow them into the realm of the inorganic beings, and to make them stay. The ultimate purpose of this is to hunt awareness, like fishermen catching fish.

The inorganic beings are said to be of two kinds, watery and fiery. Watery ones are described as 'the most annoying', dependant and possesive.

Castaneda came into contact with a watery inoganic being. Inside this inorganic being, he found that the 'mobile part', of which scouts and the Dreaming Emissary was part, where of three kinds: Round, Bell Shaped and Candle Flame Shaped. The Emissary said that it was Bell Shaped, and that the Scouts where Candle Flame Shaped. He never got to know about the round shapes.

The inorganic beings are said to be a labyrinth of tunnels, each of which is alive. The mobile part of the inorganic beings live and function in the immobile tunnels, in symbiosis.

The inorganic beings are said to be female, and they have an affinity for male energy.

References:
The Fire from Within: [cc7, ch 6]