Feral World

Feral Worlds are uncommon in the Milky Way. They are usually populated by humans, and have regressed technologically due to lack of resources over the millennia. A few are populated by indigenous Xenos barely advanced at all in culture, who still live in a primitive state.

Geography

Feral worlds take on any aspect of geography. The geography of each one is unique, ranging from worlds that have mountain ranges, deserts, seas, continents, artic zones to tropical zones. A few have limited regions to the full gamut of regional geography.

Ecosystem

Some have the bare minimum of ecosystems, and at the other end of the spectrum, full thriving ecosystems.

Ecosystem Cycles

Depends on the availability of ecosystems.

Localized Phenomena

Again this depends on the nature of the world itself.

Fauna & Flora

Depends entirely on the specific world.

Natural Resources

Again, depending on the world in question. It may be lacking any major natural resources, or be teeming with many. The only guaranteed resource is the human population.

History

Many worlds where Humanity has settled before the birth of the Imperium had thriving colonies. Then over time they devolved due to running out of resources. This would have been due to war in or near the system, the planet itself, or some kind of natural disaster happening. Maybe even some type of stellar phenomenon happened caused society to collapse.
Over time the colonies split into many different factions, and as technology failed with a regression in society, the local inhabitants had to rely only on the most basic of these to survive. The strongest would dominate the others, and primitive technology is used, to the point of not even the level the Dark Ages of Old Earth. Spears, axes, and such became the standard weapons, and these rely on anything from flint to basic iron or rough steel being used.
The societies are usually spread from tribal hunter-gatherers to primitive townships & agriculture. Usually the best are 'wealthy' and powerful in their own way, but nothing like the advanced worlds that others live on. This can lead to culture shock when the Imperium takes an interest in the world in question. Usually ruled over by a Planetary Governor who likely lives off-world, on a space station, moon, or other planet in the same system, the feral world has many benefits of it's own.
These benefits might come in many forms. Maybe its rich in certain resources like naturally occurring drugs, lethal animals that make for good hunting trophies, specific metals that can only be processed off-world. The list is endless. In every case the one thing that makes them even more valuable is the local population. Strong, tough, and resourceful, the locals are born survivors. Any who can't, die.
With this in mind, quite a few are handed over to the Adeptus Astartes directly or used to recruit a certain portion of the population as needed. This helps breed good soldiers, and some are excellent breeding grounds for resourceful Guardsmen for the Astra Militarium.

"Feral worlds can be strange places. Some are populated what the scribes would say what Holy Terra was like when we first evolved. Others are so alien to the eyes you wouldn't believe.
One thing is true though.... The locals may be primitive in the extreme, but will think nothing of ripping any of us apart with their bare hands if you piss them off. Seen it happen to a preacher that went down thinking he was immortal, and that the Emperor would protect him. Big mistake. Took us three days to be able to get his corpse back, and the equipment he took down with him. Damn fool."
Paul Arrakis. Voidborn explorator of the Rogue Trader ship 'Lance of Sol'.

Type

Planet