The Land of Faerie
The Land Beyond The Mists is a term used to describe a number of interconnected and enmeshed planes that lie parallel to, and at times intersecting, the Mundane -- also called Faerie.
These planes were originally far more enmeshed with the Mundane with travel between the planes being easily achieved by anyone who knew the way but during the Sundering of the Divine (approx. 20,000 years ago), these planes were more fully separated from our own the boundaries (also known as "The Mists")between them thickened considerably.
The separation of Faerie from the Mundane was achieved through a rare collaboration between the four seasonal courts and required some sacrifice of the magic that the Fae previously had access to. The separation was performed primarily to protect the Fair Folk from outside influence -- namely, that of beings from Outside the bounds of the Ordered Cosmos...
During that time, these planes had developed their own unique ecosystems, geography, culture and languages that while not entirely separate from those represented in the Mundane, are still considerably different to the point of seeming largely alien to any Mundanes who might find themselves lost beyond the mists.
Prior to the Waning of Magic (which began around 1200 AD) the Land of Faerie was far more accessible to mortals and travel between the Mundane and Faerie was far more common. Even in the Modern Era, there are places where the Mundane overlaps so much with Faerie that the two are essentially the same place -- generally, this is in places of deep and untouched wilderness -- or liminal spaces. Some such liminal spaces include: rest stops/truck stops, bridges, abandoned buildings (especially when overgrown), any place where land and water meet and overlap or deep underground.
There are also times when the Mists are easier to breach: dusk and dawn, during a black or blue moon, solar or lunar eclipses or cloudy/misty days when it is impossible to tell the time by position of the sun or the quality of its light.
The Seasonal Courts
The four primary planes of Faerie, are also known as the Seasonal Coruts -- Spring, Summer, Autumn and Winter. As the names would suggest, each of these planes are caught in a sort of perpetual season stasis with each at its height, with Winter and Summer at their Solstices and Spring and Autumn at their Equinox. Thus Spring and Autumn have days and nights of equal length whereas Summer and Winter have longer/shorter days and shorter/longer nights, respectively.
The four planes bleed into each other and it is possible to travel from one to the other with the proper intention and knowledge of the correct Paths. The Boundaries of each plane are also clearly marked.
Each of the Seasonal Courts has a unique climate and geography as well as flora and fauna. Summer is largely a mix of tropical and sub-tropical wetlands, jungle and temperate forest. Winter is largely tundra, boreal forest and mountains.