The Source Worlds are planets in the same solar system. There are four worlds; Shadow, Astral, Order and Natural. Each world represents a source of power, either as the actual source, or the place where the power was first discovered. Each world has terrain which is similar to, but still distinct from the others.
A natural world, similar to Earth. Several large continents, with varying amounts of life, terrain and weather conditions. It is a predominantly verdant world, with a few deserts and tundras.
Nature spirits watch the world and protect it from outside forces. These Spirits originally taught natural based magics to Druids, Shamans and others. Intelligent, animal-like and plant-like beings come from this world. Kenku, Minotaur, Kreen, Shifters, Wilden, Satyrs, Hamadryads and other animal based humanoids are predominate.
This world is very old, and other races have moved here.
A smaller world than Nature, it is fairly ordered. There is one very large continent, and the land tends to be flat. Forests grow in ordered patterns and rivers often create borders between nations. The ocean is fresh, rather than salty.
The rulers of this world called Powers or Gods, rule from the mountain tops. Baator and Mt. Celestia are two most powerful mountains from which the world is ruled. These mountains tend to be isolated behemoths, from which the gods rule. Ultimate reward and ultimate punishment are clear to each person, creating a forced harmony. Gods, demigods and Saints (often called Riders) walk this world. The Powers first taught divine magic to their worshipers. Humans, dwarves, halflings and other natural humanoids walk this world.
A great silver ocean with small island chains dot the world. Psionic powers were first developed here. Philosophies dot this world like the islands. This world has developed the Dreamscape, an alternate dimension, where the Philosophies take a nearly physical form, called Ideas. The Ideas are not truly alive, but they act as if they are. The Shardminds are natives, and created the warforged after meeting others. This is a truly ancient world.
A small world which rarely has direct light exposure, the world constantly drains life, and gives something else in return. Fungi cover the world, and it tends to have an extensive underworld as well. The world itself only has one ruler, a powerful being called the Lichwraith, The Caller or sometimes just Death. Shadow beings and Shadow Magic have their roots here.