Excerpt from "A Dissertation on the Realms," by H.R.M. Thansor.

Prologue.
"The realms are a subject that have fascinated countless souls through the course of time. The ability to travel from one world to another is an ability that many seek, but few can master. In this volume, I seek to explore and explain the realms in a manner that the common magician or witch can understand. For a moment, though, I ask you to try and view the world, the realms, as a true Realm Mage.
You, as an individual, are a single point, a star in a vast cosmos. Your physical form desires to interact with other physical forms, and so, you and an infinite number of other points come together to form the known realms. The dirt, stones, plants, animals, beasts, sentient beings, all of them are simple points trying merely to interact with each other in the whirling void.
If your form, your energy, desires to interact with a different set of points (i.e. another realm), you must cross the infinite void in order to reach that realm. Ordinarily, this would be impossible, as an individual simply can't survive in the void. However, these clusters of energy, these realms, are not stagnant. They move, they spin, and, occasionally, they brush up against each other. It is through these intersections that an individual can safely travel from one realm to another. The closest realm, colloquially known as the 'Dark Realm,' intersects with our own every single night. Others only intersect once per year, or on even rarer occasions. Still others are connected to our own realm through objects, points, that can exist in two clusters simultaneously.
As stated, I will do my best to simplify these interactions as much as possible. Allow me to expand your world... And open your mind to the many more worlds out there.