The Courts are a way of identifying a changeling's personal view of the world. The two Faerie Courts are diametrically opposed to each other, two sides of the same coin, as it were, and often work at cross purposes. Once upon a time the Courts existed in balance according to the cycle of the year. The Seelie ruled from Beltane to Samhain, reveling in the light and warmth of summertime. From Samhain to Beltane the Unseelie held sway, Ravaging Glamour where they wished during the darker half of the year. Since the Shattering, however, this ancient balance has, for the most part, been abandoned by the Fae on earth. By a centuries-old compact, the Seelie rule year-round, hoping their light and warmth will be enough to stave off the coming Endless Winter. The Unseelie revel and scheme in the shadows, playing out their Pagent, an ancient game of politics and social posturing, waiting for the time when they will once again rise to power.
The Seelie Court
Traditionally associated with summer, light, and daytime, the Seelie Court sees itself as the keeper of lore and tradition among the Kithain. Seelie changelings concern themselves with the preservation of Beauty and the pursuit of Love. Honor is their most prized possession; Seelie Fae live and die for honor. These Fae also have long memories, and a debt owed is never forgotten. They will honor their debts until such a time as they may be repaid. By the same token the Seelie hold others to their debts. Once one has been granted a favor by a Seelie lord, one can most certainly expect that lord to call the favor due some time in the future. The Seelie also see themselves as the guardians of Glamour. In these dark days sources of Glamour are few and far between, so those which exist must be protected. Glamour must be used carefully lest it be squandered and gone in time of true need. Seelie changelings spend much of their time in contact with Dreamers, those rare mortals who possess the creative spark necessary to fuel the imaginations and dreams of those around them, acting as patrons and guides. These Muses (as such Faerie patrons are known) work carefully to fan the creative spark into a flame and bring Glamour back to the Autumn world.
For all this, it should be noted that Seelie does not necessarily mean good. A Seelie paladin may slaughter a family to repay a debt or raze an entire village to cleanse the land of the taint of evil. The Fae have ever lived by their own code, which was old before Man was young. Centuries of living among mortals has brought modern changelings are closer to mortal morality than the True Fae, but they are still creatures of the Dreaming with concerns and values beyond mortal comprehension.
The Unseelie Court
Just as night defines day, the light of the Seelie Court is defined by the darkness of the Unseelie. Members of this Court have given in to the darker side of their natures, and are traditionally aligned with darkness, night, and winter. The traditions of the Seelie Court hold little meaning to the Unseelie. Change is good, for without change the world grows still, becoming stagnant and putrid. Honor means little to the Unseelie. What is the point of dueling over some trivial slight or dying for some out-dated cause? Better to take what one can, then slink into the darkness to lick one's wounds and wait for the opportunity to take revenge. As agents of chaos and change, the Unseelie are less concerned with the preservation of Glamour than the Seelie. While the Unseelie often indulge in Musing darker forms of expression, just as often they Ravage mortals - ripping the Glamour from Dreamers through a variety of unsavory tactics. There will always be more Dreamers, so why hoard the power which is the Faerie birthright?
Not all Unseelie changelings are twisted monsters. Some changelings are Unseelie simply because they don't agree with the tenets and posturing of the Seelie Court. These Unseelie typically prefer Musing to Ravaging, but are not above taking Glamour when in need.
The Shadow Court
Once the Seelie and Unseelie traded kingship of the seasons, the ancient cycle of shifting power has largely been abandoned. Even so, tradition dies hard among the Fae. Each Samhain, rulers of the Seelie Court abdicate their thrones for a single night and an Unseelie Court is elected in its place. This "Shadow Court" mocks the traditions and rulings of the Summer Fae. To most kithain, the Shadow Court is nothing more than a Halloween ritual, a time when they can indulge the darker parts of their Faerie natures without fear of reprisal.
However, some changelings whisper of more sinister dealings in Winter's darkness. The Unseelie, they say, have formed a secret alliance and work stealthily in the shadows to overthrow the Seelie Court. Rumor has it that these twisted Fae are actually working to destroy Glamour and bring about the Endless Winter. Most of these doomsayers are dangerously close to Bedlam, and saner Fae discount their rantings as the product of Glamour-induced madness. After all, what true changeling would wish to destroy the Dreaming?
Oathcircles are similar to the "adventuring party" referenced in many fantasy works, e.g., the party of dwarves in The Hobbit, with one important exception. Members of an oathcircle have sworn an oath of comradarie and mutual protection to the rest of the circle; membership in an oathcircle is serious business.
While oathcircles may be formed for any cause, the most common reason is an affirmation of true friendship. Oathcircles become changelings' Faerie families, helping one another remember who and what they truly are. Its members watch each other for signs of Bedlam, gently but firmly sending the afflicted back to his mortal life for a time whenever madness seems to take hold. When one member Forgets her true self, her oathcircle is there with Glamour to restore her memory. If an oathmate is in trouble, the rest of the circle will be there for support or rescue.