"Life can be as complex, or as simple, as you make it."
Warning! This section is for DM's only! Players should not, not, NOT read this! This section contains spoilers that will ruin the entire campaign for a Player, as it explains the secrets of Outland no Player should know from the start.
If you are a Player who's DM will be running an Outland campaign, and you read this anyway, you are a sneaky little guy! Shame on you!
Outland was designed to be "dropped" into almost any existing campaign. It can be played on its own, but works best if placed into an existing campaign setting. I have designed Outland to fit within my own campaign world of Tryll (Go to Tryll), but with minimal modifications, can be used in almost any AD&D game world.
Outland is a demiplane created by an unknown race, for the purposes of raising humans and demihumans. Not as cattle, but as spies, assassins and infiltrators. Through a careful program, these creatures, called the Hidden for now, hope to raise adventurers with a strong loyalty to Outland. Then, by arranging feigned "attacks" on Outland from various sources, the Hidden will create a great hatred in the PCs for that particular foe.
The Hidden have arranged things so that the people of Outland truly believe themselves to be tucked away in a remote valley on the game world in question. So, the people of Outland in my campaign of Tryll actually believe that they are hidden away somewhere on that planet. The vast majority have no idea that they actually live in a demiplane on the Ethereal.
For example, assume that the Hidden wish to raise the PCs to attack an elven stronghold on the Prime. The PCs grow up defending their homes from periodic raids by these "elves" (who are, of course, actually minions of the Hidden), and thereby grow to hate and despise them. When the time is right, the Hidden arrange for the PCs to have the opportunity to get their revenge, and arrange an attack on the elven stronghold.
The key to running an Outland campaign is to create a deep sense of home and belonging. The goal of the Hidden is for the characters to grow up willing to sacrifice their lives to protect Outland against the Savages (a term applied to anyone from Beyond). To this end, the PCs experience numerous adventures where they must defend their homes and families from vicious invading Savages (anyone not from Outland).
Outland is a demiplane designed to mimic a temperate environment on an average Prime plane. Basically oval in shape, Outland is about 700 miles long, and approximately 350 miles thick in the center. The sky overhead is 100 miles above the "surface". The world appears similar to an average Prime world - the sky is blue, clouds white, and there is one sun and one moon. Their are 24 hours in a day, seven days in a week, and 30 days in a month.
In Outland, the temperature is always that of early summer. Temperatures raise from 50 to 90 degrees Fahrenheit. Precipitation is common, but happens mostly at night. However, occasional rainfalls do occur during the day. The low-lying areas of Outland are sometimes subject to thick, coloured fogs, which can linger for days. These fogs are slightly pink in hue (as opposed to the standard grey), with veins of purple and red running through them. The fogs were not intentionally created by the hidden; however, they don't know how to get rid of them. Seeing as they don't seem to bother anything, these fogs are basically ignored.
Outland is ringed by a massive mountain chain (The Savage Fangs), which runs approximately 1,000-1,200 miles in length. It (along with the Aludian ocean to the south) form a barrier around Outland designed to keep the PCs from ever accidentally stumbling across the Border Ethereal. PCs who inquire as to what lies on the other side of the mountain are told that this is the domain of whatever enemy the Hidden are raising the PCs to hate. They are cautioned never to cross the mountains, for this enemy is fierce, and would certainly kill them.
There is a border of land (referred to as the Boundary), approximately 5 miles wide, between the Savage Fangs and the Border Ethereal. This border consists of flat grasslands, with nothing to block line of site. Even the grass is only inches high. This area is guarded by invisible stalkers, whose are there to make sure the PCs never cross the Boundary without permission.
This presents the DM with the key, secret life of Outland. For more generic details, you can consult the open sections of this site.