There are over one million worlds in the Imperium, all of which are inhabited by humans or human descended creatures such as beastmen and squats. The imperial administration may choose to consider these mutations as human or otherwise. In any case, they are all citizens of the Imperium - although citizenship confers no rights, only responsibilities.
The worlds of the Imperium are scattered throughout the galaxy; they are not confined to a specific area or territory. The distances involved are vast, and many human worlds will be inaccessible, or have been so in the past, due to warp storms or governmental apathy. For these reasons the Imperium includes a vast variety of cultural and technological levels. Planetary governors and other Imperial Commanders always maintain a fairly high level of personal technology, but the worlds they run may be inhabited by primitive savages, or overrun by mutants and native creatures. The worlds of the Imperium can be classified into the following broad categories:
Agricultural worlds are little more than farming planets on which a sizable part of the surface is given over to producing food for other, less fertile, worlds. They tend to be sparsely populated. The Imperial Commander of such a planet has the added responsibilities of protecting his harvests and meeting his quotas. Inter-commander rivalry often results in enemies attempting to destroy or steal crops or meat animals, often blaming raids on pirates or bandits. Such petty rivalries are of no concern to the Adeptus Terra - who only demand that quotas are met and conflict contained.
Civilized worlds are by far the most common of all the types of settlement in the Imperium. The people inhabits urban centers supplied by the planet's own natural resources and agriculture. These worlds are self-sufficient, and have reasonable, but not excessive, populations. The social and technological base varies from world to world, although access to fully developed technology is usually possible. Although these planets are civilized - in that their inhabitants live in cities - the humans that inhabit them are as bound by superstition, mysticism and barbarism as are many others in the Imperium. In the cities, sophisticated urbanities pray to the same gods and incant the same rituals as dull peasants in isolated villages. For urban warriors and technological barbarians, rationality and science are as abhorrent as to the most hide-bound rural farmer.
Death worlds are planets on which the native flora and fauna has evolved into naturally aggressive and dangerous forms. These eco-systems are finely balanced between continual destruction and lightning-fast reproduction. Humans can, and do, live on these worlds, but it is a never-ending struggle. On many death worlds it is as if the entire bio-mass of the planet were consciously motivated against human settlement - concentrating forces against intruders to destroy them. Death worlds are not usually inhabited unless there is some good reason to do so - such as outstanding mineral wealth.
Feral planets are worlds which have reverted to savagery, either because of neglect, a naturally inadequate ecosystem, or long isolation. Human groups roam the surface as wandering hunters, using primitive tools and weapons - these people have a barbaric and aggressive view of life that makes them ideal material for the Legiones Astartes. Imperial commanders on such worlds tend to live as isolated 'gods', perhaps in a single civilized city inhabited by outsiders, mostly military staff and their families. Apart from recruiting for the Space Marines, and keeping a check on emerging psykers, the Commander will usually leave his subjects alone.
Hive worlds are distinguished by vast, continent-spanning cities, often built high into the sky and deep below the ground. Their populations are enormous, and almost all food needs to be imported. A hive world rendered temporarily inaccessible through warp space will suffer a devastating famine within a very short space of time. It will become a vast catacomb of lunatics driven to excesses of anarchic, urban savagery by starvation and claustrophobia. Hive worlds are dangerous, being too large to monitor safely, and their citizens are typically unbalanced, if not utterly crazed. It has been known for the Adeptus Arbites to cull these planets in order to bring their populations down to manageable levels.
Industrial worlds are given over to industrial processes such as manufacturing and mining. They are only sparsely populated, as most work is carried out by machinery and robots. Most industrial worlds are developed only for mining and, even then, a planet must be extremely mineral-rich if the effort is to be justified. Normally, manufacturing of goods takes place on ordinary, inhabited planets, because the costs and hazards of inter-stellar flight are considerable.
Medieval worlds are feral planets in which a level of medieval technology has been reached, and the culture has stabilized. Imperial Commanders of such worlds will often stand aside from their subjects, and may even remain aloof in orbital space-stations or on a nearby moon. Medieval worlds are self-sufficient, but are of little use to the Imperium. The true position of their place in the universe may constitute something of a culture shock to the inhabitants, a factor which makes them poor material for imperial service - although selective recruitment into the Space Marines is possible. Control of psykers has to be maintained - but this can be achieved in a clandestine manner; by infiltrating religious and social bodies, or by more blatant means such as kidnapping and assassination.
Paradise planets are worlds of outstanding natural beauty and abundance. Rather than develop all of these planets, some are retained in an almost completely natural state and used as recreational bases for imperial servants. On such planets, warriors may train their minds and bodies for war, studying arcane battle-philosophy and practicing martial arts.
Research stations are recently inhabited, often newly accessible planets in the process of being assessed for development and full settlement. To this end, they initially become research stations, small farming centers, experimental settlements, test mines, etc. Mostly they are just wilderness - a whole planet awaiting exploration.
From Warhammer 40,000: Rogue Trader