"Anything breeds with anything."
You name the race, it exists as a sentient species on the world. Cross-breeds are also everywhere, and anthropomorphic versions of most anything are common. There are even enclaves (some relatively peaceful) of sentient undead, naturalized elementals and the like, though these are often small.
This setting will use the Greyhawk deities. Basically, any deities from this document or from any of the non-campaign-specific WotC books (such as these added in "Libris Mortis").
Due to the nature of the setting, few deities have maintained a purely one-race base of worship -- since few people are pure-blooded anything. Consider any racial deities to be general deities with a favoring towards those with that racial heritage.
The known world is an archipelago. There are uncountable islands close and far apart covering the seas of differing sizes. The coasts of the islands are where the population tends to persist. As one gets farther from the coasts, the interior of the islands rapidly become more feral and wild. The interior of larger islands is lethal enough that even druids have difficulty surviving there.
This provides some boons, however. Nature regrows quickly. This allows for the rapid replenishment of wood resources (which is quite the blessing to the shipbuilders.) Wild food also is plentiful, though expensive: those who press their luck hunting food in the feral wilds do not have long lives. All resources, other than wood are recycled as much as possible, especially stone, metal and the like because they last longer than wood and are hard to harvest.
The same is also true in the other direction. The farther one goes to see away from a coast, the larger, wilder and more aggressive the sea life become; the waves themselves become angrier. This makes for most sea travel to take the safer round-about path from island to island rather than the direct route that would cross the wild seas.
Because of the plentiful natural resources from coastal interiors, and the need for the populous to be concentrated at the coasts, many of the larger islands are very urbanized along their coastlines. The urbanization, however, is usually rather slum-like. Not all islands are urbanized. Some do remain rural, and there are plenty that are not yet colonized by the known intelligent species.
The climates of the world do not follow the normal earth standard for a warm wet equator and a cold dry polar region. Climates also can change as is common for a fantasy world. One island can be a desert island, with the next one in the local group being a jungle. The cardinal directions are known and steady, and used for navigation.
As part of character creation, each player helps with world building by creating an island for the game world.
Each island essentially has its own culture. Nearly all islands stand on their own. Few political structures span beyond one (or several very close) islands.
Many people keep private gardens (though, usually, small because of urbanization) to supplement their food. This is an arduous task, however (weeds grow at an amazing pace, of course). The vast majority of the food in the world is provided by governing structures. Secular governments organize their people (the more, the better) to maintain fields and to form hunting, foraging and fishing parties that are large enough to maintain a modicum of safety. Religious governments do much of the same, but also maintain a large body of clergy for the purposes of magically sustaining their people.
In either case, the people often find themselves entirely beholden to the ruling structure under which they live. It should be noted that the formal governing structure of an area may not actually be the one that takes care of the people and earns their respect. Even evil societies tend to operate generally in a similar way. Chaotic societies are rarely are capable of sustaining themselves on larger islands, but there are plenty of smaller ones that lead a less rigid way of life.
The larger, more populous and urbanized islands generally are those with stronger religious orders. More divinely-conjured food is needed, so the clerical caste grows. This, in turn, creates an increased supply of healing, which helps to keep a healthier population that itself grows more.
Because of the necessity of large power structures on most islands, many become monocultures.
According to general knowledge, there is nothing special about the planar arrangement of this plane compared to the others. It is just another crystal shard in the infinite prime material plane. The standard Planescape-style cosmology is used.
There are some known issues with planar connectivity, though. Scholars don't agree as to the cause, but some most spells involving planar links tend to be weakened slightly. Extraplanar summoning spells, for example, take longer than normal for their effects to happen. Deity-contacting divinations have delayed effects (ie, the answers may come in your sleep a day later). (See Alterations for how these changes happen mechanically.)
Temporary Nature of Objects
All objects in the world are temporary. Everything decays and deteriorates at a faster rate in this world than elsewhere. Writings (especially maps and sea charts), for example, fade and disappear within a generation, often faster. It takes a lot of constant effort to maintain and preserve decent libraries of any size. Buildings crumble and erode in centuries rather than millennia. This keeps life hard for everyone, but especially brutal to the laborers of the world. As mentioned in the geography section, this is more prevalent the farther one is from the cost. The longest-lasting, oldest structures are at the coasts. Shanty towns closest to the interior of the islands need to be repaired continually and weeding is a daily task as nature struggles to reclaim the land.
Of particular note, most magically-imbued items lose their potency as soon as their creator dies. The life essence of the creator of the item is intrinsically entwined with the item itself. Even magic items meant to be temporary (such as potions and scrolls) lose their enchantment once their creator dies. It should be obvious that this makes your source of magic enchantments an important decision. There' also an added inherent risk with acquiring magic items from others, and it also intrinsically makes you want to keep your favorite enchanter alive. Targeting well-known magic item enchanters is a common tactic for islands waging war with one another. See Rules.
There is a weird occurrence that sometimes happens: things sometimes transmute at random. Sages have no idea why, nor have they found a pattern as to when or how it happens. The only thing known is that sometimes a dog will turn into a boulder, a glass of water will turn into a glass of molten lead, or a house will suddenly turn into a mansion.
Sages categorize another phenomenon the same as spontaneous transmutations: blooms and blights. Sometimes the rapid natural cycle jumps even more than normal and entire fields will will green in a week, or almost ripe fields will whither and die overnight.
The only thing truly known about this is that this happened much more often around alchemy, which is one of the reasons the art never grew terribly strong in this world.
There are no firearms in this world. Alchemy is also of very limited use. Wood and other vegetable materials are abundant. The various special materials abound. Ships are slightly more advanced than D&D standard, though they're still not age of exploration quality, though large ships aren't too common because most ships try to stay close to land because of the wild seas. Review Sources.
Just because of the decay of everything does not mean that the world is low-tech. Knowledge can be preserved with good, strong oral traditions. There also tend to be large orders of scribes in the world continually copying all writings as rapidly as possible to keep ahead of the decay of both old and new information. Apprenticeship systems also preserve and transmit information in an immaterial way.