The world is dominated by elemental forces - great lava flows and lakes of fire; massive thunderstorms and floating islands; raging rivers and huge tidal waves; strange rock formations and unimaginably tall mountains, with frequent earthquakes.
The native people of the world are genasi, with each of the four elemental sub-races present. For millenia they lived in harmony with each other and then a thousand years ago were invaded from the Astral Sea by the gith - first the githyanki came raiding, then the githzerai hunting them. Some catastrophe occurred that separated the gith from the Astral Sea and they were stranded. Over the centuries they built up their own empires and each sought dominion in different ways over the genasi and the world, although eventually the githyanki gained dominion, ruling over all.
The githyanki serve their immortal queen, Izkeroth, a daughter of Vlaakith, who rules her empire through the power of her red dragons, illithid slaves, and githyanki warriors. The githzerai live in loosely allied secret monasteries, evading the githyanki as best they can, and seeking alliance with the enslaved genasi.
Other races exist: the aarakocra of the flying isles, and the elusive aasimar who live in high mountains, as well as the green mottled-skin goliath in the great wood lands, and perhaps others. It is believed that they, as well as other creatures not native to the world, have come through via portals from other worlds which open seemingly randomly, perhaps due to whatever magic that stranded the gith.
The githzerai are, of course, often monks, but sometimes warlocks, seeking to understand and harness the secret powers of the universe. The githyanki are often fighters, psionic-endowed via feats. The goliath are known for their druidic powers, the warriors being rangers. The aarakocra serve their great deity the Sun Phoenix - the living embodiment of the sun - as paladins. Aasimar are known for their love of music and are often bards, playing mystical music, but are also natural sorcerers, imbued with magical energy (and all aasimar PCs must multi-class and take at least every third level as sorcerers). Genasi are more diverse, although fire genasi tend towards wizards who specialize in fire magic, likewise with the other sub-races.
The inhabited land is like a great wheel, with the githyanki ruling the middle and the further out, the more wild the elements. Beyond the central region is the great forest which gives way to desert in the south and great lakes and mountains of fire. To the north, wild waters and icy seas. To the east, air (and sky isles), and to the west the great rock lands and mountains.
Alright, that's it for now.