There are usually said to be four realms of power: the Divine (God, angels, and faith), the Infernal (the Devil and demons), Magic (mystical workings and mysterious beings) and Faerie (the unknown world of the fey). The Mundane realm of the natural world and of non-magi is often described as a realm of its own, but not in the same context of the other, fundamentally mystical realms; rather, the realm of the Mundane is characterized by its lack of direct influence from the four realms of power. Thus, the Mundane can be said to be a realm, but it is decidedly not one of the four realms of power.
While the nature of the realms of the Divine and Infernal are relatively clear, being as they are rooted in familiar Judeo-Christian themes and legends, the boundary between the realms of Magic and Faerie is frequently vague, with the realm of Faerie often seeming identical to the more familiar realm of Magic save that it is dominated by faeries rather than by magi. Later editions of the game (those not published by White Wolf) eventually clarified that the realm of Magic was one that presented a more perfect vision of the Mundane that functioned according to more perfect rules, while the realm of Faerie was one that was spontaneous and was at least in part drawn from human imagination. Thus, beings of the realm of Magic wouldn't necessarily bear either sympathy or animus towards humans, whereas faeries would often develop a keen interest in the mortals they encountered, be it to their gain or to their detriment (or both). This did have the side effect of reclassifying some beings presented in earlier sourcebooks from the realm of Magic to that of Faerie, such as deities and similar beings from real-world myths that had borne a great deal of interest in meddling with humankind.
With the publication of Ars Magica Third Edition, Ars Magica began to reflect the influence of the World of Darkness and specifically of Mage: The Ascension, just as the World of Darkness represented the (possible) future of Mythic Europe. Thus, a fifth realm of power was introduced: the realm of Reason, which represented the logical and decidedly non-magical side of human thinking, and which paralleled the Consensus enforced by the Technocracy (or, more pertinently, by the Order of Reason, which had yet to be introduced when Ars Magica Third Edition was published). The power of Reason begat True Reason, a trait that broadly mirrored True Faith save that it countered the power of the four traditional realms of power (including Hermetic magic) rather than working miracles against the non-Divine. The realm of Reason proved to be controversial at best with Ars Magica players, who saw it as an unwanted intrusion of Mage's more cynical postmodern themes into the decidedly dissimilar world of Mythic Europe. Additionally, Reason's scholastic bent overlapped awkwardly with the traditional areas in which the realms of Magic and the Divine held sway - and as the realm of Magic was the realm of magi and thus of the players' characters, this created no small amount of confusion. Consequently, Reason was firmly dropped from the game in later (post-White Wolf) editions of the game.
Three sourcebooks for the realms were published by White Wolf: Pax Dei covered the realm of the Divine, The Maleficium the realm of the Infernal, and Faeries the realms of the fey folk. No book published by White Wolf (or by Lion Rampant, before the merger and name change) focused specifically on the realm of Magic, although a number of books provided a wealth of information on the magi, their workings, and the world they had made for themselves.