My world tends to be more Greyhawk than Forgotten Realm, more John Carter than Lord of the Rings so alignment is about the cosmic war between order and chaos (most exemplified by the Blood War in the D&D mythos). Lifting this from Dungeon Crawl Classic as this is a good definition of the three-axis of alignment.
In the beginning, there was the Void, where the Old Ones dreamed. In their dreams were Law and Chaos, inherent forces of unity and entropy. Through endless opposition, these forces of unity and entropy elected champions who became gods, who in turn formed planes of existence that reflected their principles. On one such plane resides your trivial existence, tiny next to the vastness of Aéreth, even tinier next to the vastness of the cosmos. But you are connected back to the greater universe and the endless struggle by a fundamental choice: do you back the forces of Law or the forces of Chaos?
Alignment is a choice of values. In its simplest form, it determines behavior. In higher forms, it determines allegiance to a cosmic force. Characters choose an alignment at 0 levels, and this choice determines their options for the rest of their lives.
Alignment functions on many levels, but there are two primary extremes: lawful and chaotic, with the balance of neutrality between. A character chooses one of these three alignments at 0 level.
Lawful characters believe fundamentally in unity and prioritize the values of mankind: order, authority, loyalty, and charity. They support organized institutions and “do what is right.” They have a moral conscience that points them toward the appropriate action. Fundamentally, lawful characters choose the path of mankind over the path of supernatural dominance. At higher levels, lawful characters find themselves interacting with celestials, angels, demi-gods, and powerful Lords of Law. In mundane life, there are many shades of lawful behavior, and not all lawful characters agree on the same course of action at any time; though they invariably unite when mankind is threatened by outside forces.
Chaotic characters believe fundamentally in entropy and seek constantly to undermine or rule those around them. They are willing to disrupt the natural order of things – including established governments, guilds, and relationships – if they see a material benefit in doing so. They are open to agreements with supernatural powers, even if such agreements risk the primacy of man by allowing strange beings into the material plane. Fundamentally, chaotic characters choose the path of greatest personal power and success over any greater principle. At higher levels, chaotic characters find themselves aligned with demons and devils, sinister monsters, extraplanar creatures, and the supernatural Chaos Lords. In mundane life, chaotic behavior covers a wide spectrum of chicanery, subterfuge, aggression, and power politics, and chaotic characters are always looking for an advantage over their peers.
Neutral characters have not taken a stand between Law and Chaos. Neutrality is the balance of nature, the timelessness of eternity, and the nothingness of space. It can also reflect the neutrality of those who came before Law and Chaos: the Old Ones, the great Cthulhu, and the empty Void, and the emptiness of the time before gods. Neutrality between Law and Chaos can reflect a measured morality – balancing costs and benefits, without strong principles one way or the other. It can also reflect ambivalence or indifference. Fundamentally, neutral characters make choices based not on loyalty, values, or self-advantage, but by evaluating each and every opportunity that comes along. At higher levels, neutral characters find themselves aligned with elementals, extraplanar un-dead, and astral and ethereal beings.
The eternal struggle between Law and Chaos is real. Gods and demons battle on other planes for superiority and the actions of man give those entities power. Make your choice carefully, for it will become increasingly important as you become more powerful.