Central to Theosophy is the theme (also found in the Hermetic Order of the Golden Dawn, another 19th century occult revival) of a Hierarchy of perfected men and women who have finished their cycles of reincarnation and material evolution (although there are still higher stages to be attained, evolution being infinite) but have chosen to remain here on the Earth to serve humanity and guide the Earth's spiritual evolution (essentially the Mahayanist Bodhisattva Ideal in new guise). All Theosophists claim that their teachings were derived directly from occult masters. Theosophists like Blavatsky and Leadbeater claimed to be in touch with, and receive materialised letters from actual physical masters residing in Tibet. Blavatsky's master was said to be Kut Humi, (portrait above, referred to by Leadbeater as a reincarnation of Pythagoras and of Francis of Assisi). Heterodox teachers like Alice Bailey these became telepathic transmissions (channelling), and finally the Tibetans were replaced or supplanted by other masters, or masters without a physical locality.
The work of individual members of the Hierarchy is described in books such as C. W. Leadbeater's The Masters and the Path; Alice A. Bailey's The Externalisation of the Hierarchy, and Asger Lorentsen's Man's Inner Structure.
Alice Bailey in particular gives a striking account of the gross physical objectivity of her master; it is upto historians and scholars to check all the documents, records, and papers to determine if any of this can be indepeendently and empirically confirmed (personally I think this is more like the superficially persusasive accounts by Adamski, who likewise had Masters, only he located them on other planets in our solar system, something we now know to be impossible because of discoveries regarding the inhospitrability of other worlds)
The basic Theosophical/Baileyian cosmology of the masters might be represented as follows. The leader of the Earth's spiritual Hierarchy and embodies the heart centre of this planet is the Sanat Kumara ("Eternal Youth") of Blavatsky, who was supplanted in post-Blavatsky Theosophy by the Maitreya, described as a re-incarnation as Krishna, and is the Christ principle that (in the docetic system of Theosophy) was the real inspiration behind Christianity, overshadowing Jesus during the last three years of his life in Palestine. This particular christology was adopted by Rudolf Steiner, who elaborated it in great detail.
In the Theosophical scheme of evolution, souls constitute a ladder of evolution (a kind of temporalisation of the "Great Chain of Being", and we are destined to progress through higher and higher stages of the spiritual hierarchy and even beyond. This chain of being links the lowest stages of consciousness and evolution (e.g. the mineral kingdom) with the highest. Here is one representation, based in part on the Alice Bailey school. Beyond the Sanat Kumara stage (the Logos of the Earth) there are of course higher, cosmic stages of divinity.
7) Council Chamber of Sanat Kumara
These seven stages can also be matched (through the principle of correspondence) with the seven planes, seven principles, seven rounds, seven root races, seven rays (Alice Bailey) etc.
It should be pointed out that whereas most orthodox and heterodox schools and traditions of theosophy are pretty conservative regarding the Hierarchy, which is unanimously perceived as being ascended human masters, Rudolph Steiner presents a radical interpretation which harkens back to the neoplatonic angelology of Dionysius.
Although Theosophists and post-Theosophical individuals and groups like Alice Bailey, Summit Lighthouse, and some UFO religions argue for the objective existence of masters, K. Paul Johnson however very plausibly argues for a more secular interpretation, that Blavatsky based her Masters on actual historical figures she encountered in her various travels, see The Masters Revealed: Madame Blavatsky and the Myth of the Great White Lodge. The whole thing later blew out of control as the myth took on a life of her own, something that even HPB herself regretted. My own take on this is that there are two distinct realities; one the physical individuals that Blavatsky used to create the myth of the masters, the other actual occult and esoteric forces, gods or attractors, or if you are a Jungian you could say archetypes, taht then used this thoughtform as a vehicle of their manifestation in the human psyche (the work and influence of Alice Bailey being a classic example). Most New Age channelling however is of a lesser nature, channelled entities tend to be very trivial. There are a few exceptions like Seth, but I mean the bulk of them like Ramtha, Mafu, and others.