From Lexicanum

Imperial planets are categorized by the Imperium into several broad classes according to their function, type of civilization and technological level. These classes include:

Alpha or Agri-World
Gamma or Civilised World
Delta or Dead World
Delta Tau or Death World
Eta or Hive World
Mu or Feudal World
Rho or Research Station
Phi or Forge World
Phi Lambda or Feral World

Some areas of the Galaxy have many worlds of a similar state. An example of this are the worlds of the Halo Zone at the furthermost reaches of the Galaxy, which consists mostly of dead planets orbiting very old stars. Worlds near the galactic core are also almost invariably dead or barren.

Sources

• Warhammer 40,000 3rd Edition Rulebook

Worlds of the Galaxy

Imperium	Adeptus Astartes Homeworld • Agri World • Cardinal World • Civilised World • Dead World • Death World • Desert World • Feral World • Feudal World • Forge World • Hive World • Ice World • Industrial World • Jungle World • Knight World • Mining World • Ocean World • Paradise World • Penal World • Research Station • Shrine World
Xeno	Xenos World • Daemon World • Exodite World • Tomb World • Maiden World • Crone World • Ork World • Tau Empire • Craftworld