The Prime[]
E'n sits in the center of the Prime Material, the true nexus of the multiverse. All other planes of existence expand out from there. It can be said that the number of planes are infinite, but in truth, it is simply all in how you look at it, and ultimately, counting planes has little meaning.
Think of the Prime as an axis, a metal rod. From this rod, attach a cuff that extends away from the rod to a series of weights and balances at the end. The cuff and extension is a coterminous plane, one that maintains many connections to the Prime and is the easiest to access. The weights at the end of the extension are the outer planes, divorced in space and time from the Prime, and maintaining their own order.
The Essential Path[]
The first extended apparatus connected to the Prime is the Essential Path. Sometimes referred to as the Inner Planes, they are connections to the basic elements of reality. They exist on the back of the ether, the substrate of reality. The Ethereal Plane is the coterminous plane, and extends to the Elemental Regions. This is usually represented as an octahedron with six vertices for Air, Earth, Fire, Water, Light and Darkness (some traditions refer to the last two as Life and Death, or Positive and Negative Energy), but the truth is that none of these planes is fully distinct, and regions between the planes can be found which more closely represent such concepts as Smoke, Magma, Lightning, and so forth. One of my favorite is a spot where Fire and Water meet to form Alcohol (although approach the Darkness from there, and it becomes Oil). No one can be said to rule these areas, and they remain primitive but useful resources.
The Moral Path[]
The second depends on enlightened action, and is referred to as the Moral Path. The coterminous plane here is based on the stellar charting of good and evil action and is referred to as the Astral Plane. The Astral Plane extends toward a group of outer planes commonly referred to as, well, the Outer Planes. There are seven major planes or vertices here, based on the seven archetypes. However, much like a conic section, most traditions maintain greater links between the planes as just a small slice of the whole structure instead of each plane separate and indivisible. In this way, the Venzan pantheon shows greater links between the Consort's Realm and the Storm's Realm than, say, between the Venzan Storm's Realm and the Orcish Storm's Realm.
The Hidden Path[]
The third apparatus is quite mysterious, and is referred to as the Hidden Path. This is the realm of fey, and the Twilight Realm is the coterminous plane, leading to a realm known as the Other World. There is much unknown about this arrangement. However, there have been times in which the connection has been stronger and weaker, and it is not known what this means.
Because of the still rather experimental and uncertain nature of the Hidden Path, there are hypotheses that there may be another unknown apparatus out there: the Fourth Path. We can only guess what this Path is like, but the difficulty of connecting to the Western Continent may be due to connections to the Fourth Path.
Magic and the Multiverse[]
Magic exists throughout the multiverse, where it can be manipulated most readily in the Prime Material. Although some magic can be pulled and shaped directly from the fabric of the Prime itself, most magic involves infusion from the Outer Planes. Ranger magic is light and dependent on the ranger's connection to the world around him, and thus is also Prime magic. The monk's supernatural connection to ki is also felt to be a manifestation of Prime magic, and it fuels a number of other supernatural abilities.
The Essential Path does not have rulers acting as magical gatekeepers. Instead, magic simply follows natural law based on the essences pulled. The understanding of this sort of magic requires complex theory and rote memorization, if there is no natural proclivity. Most arcane magic is fueled along the Essential Path, especially in the case of wizards, alchemists, and many sorcerers.
The Moral Path does not require adherence to natural law, but instead requires gaining the favor of various beings. Stringent moral codes and fealty to supernatural forces are key to using magic along the Moral Path. As a result, most divine magic stems from the Moral Path. Clerics, oracles, inquisitors, and paladins are clear examples of Moral Path magic. Most Druids also swear fealty to a deity, and as such use the Moral Path for their spells as well. Infernal, Celestial, and Abyssal sorcerers might indeed also use Moral Path magic, even though it interacts as arcane.
The Hidden Path has not been well studied by many sages, but it appears that passion, emotion, and unfettered wild places are key to tapping into the Hidden Path. It has not been proven, but it is believed that bards' magic, although in many ways similar to arcane magic, does in fact make use of the Hidden Path. Fey, Dreamspun, and Shadow sorcerers as well are believed to use the Path, but it is also possible that they use a combination of the Hidden and Essential Paths. Most witches' familiars also travel along the Hidden Path, although the occasional familiar has an Essential or Moral source. The occasional druid, avoiding fealty to any gods, relies on the Hidden Path and wild places for her power.
Summoners pull energies from many sources in crafting their spells and eidolons, and can be seen using a combination of Prime, Essential, Moral, and Hidden Path magic.
As for the Fourth Path (called by some as the Far Path), there is nothing to know about it outside of theorizing. If Fourth Path Theory is correct, then it interferes with the other three paths somehow, preventing normal magic from functioning, or twisting it in ways that no one can possibly conceive. A competing theory is that Aberrant and Starsoul sorcerers might already access the Far Path, but if that is the case, the Far Path is much more similar to the other paths.