Planets

Civilized worlds
A civilized world is a planet whose development has been allowed to happen (for some time) "naturally", without any specific purpose to fulfil. These worlds are generally self-sufficient in terms of food-supply and have varying manufacturing and industrial capabilities. Many may be equivalent in culture and population levels to modern-day Earth.

Death worlds
Death world is a term used by the Imperium to designate a planet with a climate, terrain, or ecosystem that is highly dangerous to human life. Nonetheless, many of these worlds have large human settlements, notable for the strength and self-reliance of their people. Many of the people of these worlds are inducted into the Imperial Guard, or recruited by the Space Marines, a fact which is often the sole reason for the continued habitation of those worlds. Notable death worlds include Catachan and Fenris.

Death worlds and the Tyranids
It has been theorized by Imperial biologists that many death worlds are the result of seeding by ancient advance fleets of the Tyranids. Many of the monstrous creatures inhabiting these worlds, such as the Kraken of Fenris and the Catachan Devil have many features in common with Tyranid organisms, and may in fact be descended from them.

Feral and medieval worlds
These are worlds into which the Imperium has not seen fit to introduce most modern technology, although medical technology is often an exception. Feral worlds are defined by a population living in hunter-gatherer societies and having technology equivalent to Earth's Stone Age or Bronze Age societies. Medieval worlds are somewhat more advanced, having progressed as far as to discover gunpowder, and having more widespread farming. Generally, worlds of either of these classes will have little interference from the Imperium and low planetary tithes.

Governors of Feral and Medieval worlds will generally live on a space station in orbit, only descending to the surface to deal with heresy and mutation.

Many Space Marine Chapters prefer to recruit from feral worlds, since such environments tend to produce warrior-quality men.

Forge worlds
Forge world is a term for the numerous worlds that are directly controlled by the Adeptus Mechanicus. All have in common that they are completely dedicated to the manufacture of the various machines of the Imperium, the pursuit for and preservation of (ancient) knowledge and the worship of the Machine God. Because of the Adeptus Mechanicus' monopoly on technical knowledge and expertise, the forge worlds are the Imperium's primary source of all kinds of hardware: from farming equipment to war machines such as tanks, fighters, or even Titans. Ancient pacts between the Adeptus Mechanicus and other worlds and institutions of the Imperium oblige the various forge worlds to supply other worlds and the various military arms of the Imperium, such as the Imperial Guard.

The surface of a forge world is normally completely covered in massive factory complexes that stretch across the horizon. Its ecosystem has been completely destroyed. The air is saturated with toxic gases and rivers flow with toxic runoff from factories. In many cases, even seas and oceans have been dried up to make room for more factories. However, the sheer output benefits the Imperium as a whole.

While there are hundreds of forge worlds in the Imperium, one of the best-known is Mars, home of the Adeptus Mechanicus' political and spiritual head, the Fabricator-General of Mars; and the Imperium's first forge world. Other forge worlds include Ryza, known for its advanced plasma technology; Gryphonne IV, home of the Legio Gryphonnicus; Agripinaa, a primary supplier of Cadia; Phaeton, home of the Leman Russ MBT; and Urdesh, major forge world within the Sabbat Worlds.

Forge worlds are mostly independent from the rest of the Imperium and have their own armed forces. Each of these worlds is home to one of the Titan Legions, supported by legions of Skitarii, the Adeptus Mechanicus' cybernetically enhanced soldiers.

Hive worlds
Hive worlds are worlds that have an extremely high population, which is confined in massive arcologies called "hives", each of which is essentially an individual nation occupying a single massive hive city. Of the species in the Warhammer 40,000 universe, only the humans of the Imperium are known to live on such worlds. The world outside the hives is usually heavily polluted and desolate, and most hivers live their entire lives without ever having seen the outside of the tunnel network of their hive. Hive worlds often possess extensive manufacturing districts. It has been said that the sacrifice of over a million soldiers is worth "one day's hive world production" in weapons and armour.

Perhaps even more valuable is what at first glance seems to be a byproduct of the monolithic city's design. The population of any given world* approximately doubles every 100 years. With each hive housing between 10 - 100 billion people and 5 - 20 hives per planet the sheer number of citizens is staggering. And each of those citizens is a potential soldier for the Emperor's already unmatchably vast armies. Hives manufacture far more than mere steel and silica, they are vast factories for the most useful possible resource: people. It is no accident that hive worlds contribute the vast bulk of the recruits for the Imperial Guard. The often violent gangland lifestyle which most residents are forced to live is also semi-deliberate. Almost every recruit will already know how to handle a gun. Hive worlds also serve to populate newly discovered planets. Imperial citizens are gathered from various hive worlds (willingly or unwillingly) and shipped off to distant colonies. Examples of this include Medusa V and Armageddon.

In common with most other Imperial worlds, hive worlds are often based on a very obvious class system, with a ruling class and a working class, although with populations so tightly packed there develops a lower class that become violent street gangs. As can be expected, the upper classes are situated in the affluent upper areas of the hive, whilst the middle classes are situated in the middle areas, whilst the worker classes are packed together in the lower areas. The very bottom sections are often areas where the underclasses and criminals are sent to be forgotten about and obviously anarchy rules.

Some extensively developed hive worlds do not even simply consist of various enclosed arcologies surrounded by wasteland, jungle, ice, or plains. These hive worlds are completely urbanized and stacked with hundreds of layers of arcologies, covering the entirety of the planet, effectively being an Ecumenopolis. Holy Terra is an example of this "super hive world".

Notable hive worlds include Holy Terra (where the whole planet is one hive), Armageddon, Necromunda and Verghast (from the Gaunt's Ghosts novels).

Cardinal worlds and Shrine worlds
Cardinal worlds are planets ruled directly by the Ecclesiarchy and are completely dedicated to the worship of the Immortal Emperor according to the tenets of the Cult Imperialis, with sanctuaries potentially covering entire continents. Aside from being a center of the Ecclesiarchy's power and a destination for countless pilgrims, they also serve as a base of operations for many orders of the Adepta Sororitas. One example is Ophelia VII, whose spiritual significance is second only to that of Holy Terra.

Shrine worlds are worlds that are heavily focused on the worship of one or more Imperial Saints. A shrine world can also be considered to fall into another category, for example Hagia is an agri-world while Herodor is a hive world. They will have a strong relationship with the saint in question, such as being the world of his/her birth or death or the site of a major miracle.

Notable Shrine worlds include: Hagia (birthplace of Saint Sabbat) and Herodor (where Saint Sabbat rested while recovering from battle)

Dead worlds
Barren worlds that are virtually lifeless, unable to support any native or imported lifeforms. Dead worlds occur naturally, as the unforgiving conditions of the universe mean the majority of planets are uninhabitable. Some, however, have been created in the long history of the Imperium by catastrophic events, including an exterminatus or a successful Tyranid invasion.

Ironically, some planets classified as dead worlds have been settled by various groups specifically for their properties. For example, several Space Marine fortress-monasteries are located on dead worlds because of their harsh conditions, desolation and defensive opportunities. Dead Worlds that appear to be natural are often Tomb Worlds for the Necrons.

Tomb Worlds
Tomb Worlds are worlds that have Necron stasis tombs located on them. Such worlds are often devoid of all life, the only known exceptions being Lorn V and Kronus.