The Cosmic Axes

"If a modron can follow its programming in Limbo, then visiting the lower planes is no excuse to behave like an asshole."
- Yasmin, Planeswalker

For as long as people have travelled, such travellers have made note of the shifting characters of the Realms relative to their position on the Wheel. These characters can be generalized into four broadly-defined forces: Law and Chaos, Positivity and Negativity. A thing's relationship to these axes is referred to as its Alignment.

Individual Alignments

Scholars of the cosmic forces often scoff at the idea of a personal alignment. Because individuals can move around the wheel and maintain their alignment, they argue that a "personal alignment" is merely one way of explaining your values and desires.
Whether or not this is true, or whether personal alignment is simply more mobile than the realms is debatable. What is more certain is that the relationship between personal alignment and realm alignment is tenuous at best.
The alignment of one's home realm plays a role in a person's individual alignment only as a cultural influence.
Just as a person raised in a musical family stands a better chance of developing a talent and love for music, so a person raised in a Lawful realm is more likely to value Lawful traits. But it does not significantly outweigh other factors such as family values, role models, personal experience, interests, and other contributors.

Realm Alignments

There is no doubt that each Realm has its own "personality" which is heavily influenced by its position between the cosmic axes. In fact, whether the Realms are shaped by the axes, or the axes are simply a way of describing the Realms is a matter of debate.
The cosmic pulls on a realm can manifest in several ways. Most notably, the landscape is usually a physical reflection of the Realm's alignment. The destructive volcanoes of negatively-aligned Gehenna, or the arrow-straight rivers of lawful Arcadia, for instance.
Realms can also show their alignment in subtler ways. Some realms change the manifestation of magic—for instance, healing spells have been noted as more effective in Positive Realms. Realms can also "Condition" a behaviour within their borders. For instance, creatures who commit violence in the slightly-negative Realm of Acheron have reported an exhilarating rush of energy.

Law and Chaos

The Realms nearer the western edges of the world are distinguished by their Lawfulness. The force of Law shapes these Realms and their Realm Magic. It manifests differently between and within each Realm.
In general, Lawfulness can be thought of as a greater degree of order, control, and stability. On the other hand, Chaotic realms are characterized by their disorder, spontaneity, and changeable natures.

Positivity and Negativity

In older or more subjective texts, these latter may be referred to as Good and Evil. Relatively recently, the terms "Positivity" and "Negativity" gained prevalence, especially among academics.
Northern realms are pulled by the forces of Positivity, which promotes vitality of living things and is associated with creation, safety, and shared prosperity. For all creatures but the undead, Positive realms tend to be easier to survive in.
The southern or "lower" half of the Wheel feels the pull of Negativity more. Negativity is associated with destruction, conflict, and danger. Realms defined by the Negativity axes tend to be hostile to living creatures.

Type

Metaphysical, Supernatural