What are Changelings?

Changelings, or fae, are the creatures of human dreams, given shape and self-awareness.

Long ago, when people believed more strongly in magic, fae lived here in the world, drawing energy or Glamour from the belief of humans. However, they have their own world, also composed of glamour, which once was open to mortal realms. This world is known as The Dreaming.
As humans adopted technology and lost their interest in magic, fae's ability to live in the mortal world diminished. Some retreated deep into The Dreaming, to their own country of Arcadia, never to return. These were mostly the kith known as Sidhe, who consider themselves changeling nobility.
Here's what Arcadia might look like:

Others created footholds for themselves by taking up habitation in human bodies, much as a soul or spirit is thought to live in a body. As these changelings grow up, they undergo a Chrysalis, or awakening to their fae nature. They may express their fae selves in any of a number of ways ... artistic pursuits, crafts, technology. As long as they can bear their supernatural split selves, they share both human and fae heritage and can even consciously visit The Dreaming.

Fae possess Glamour-fueled magical powers, though those powers are Dream-stuff and tend only to work on those who believe in them. A cynic might say changelings' powers are a combination of creativity, mental illness, and placebo effect.

Changelings can collect glamour through the imaginative activity of humans; typically, this is thought of as the production of art, though crafts and technology can also produce glamour.
Fae nature is in opposition to Banality, or everyday life. Changelings tend to dissipate as they grow up and accumulate Banality. However, if they lose too much banality, they may go insane -- they do spend a lot of time, after all, in a world the people around them can't see. (A psychologist would likely diagnose a changeling with paranoid schizophrenia ... )
At war with banality:

Changelings, being derived from pagan beliefs -- Irish myth and religion, with elements of Greek, African, and Native American belief -- also have a connection to the Wild. This isn't expressed in game terms the way it is for were-creatures; however, it's worth keeping in mind. Arcadia is typically depicted as a rural or woodland court, and The Dreaming in general is envisioned as a forest.