A Forge World is one where the majority of the planets solid ground surface is covered in machine factories and production centres. There is also a requirement for forge world status regarding the gross planetary output.
The population of a planet must be huge to provide the labour to work the millions of machines producing the vehicles, weapons and other materials necessary for the success and expansion of the Imperium. It can be anything from one million to more than fifteen billion. This may be high for a planet but it does not even approach the largest of the Hive Worlds but the production from a hive world is much reduced when compared with a forge world.
Hive Worlds are the domains of the Adeptus Mechanicus and Mars is the home of this organisation. Because of their importance and the huge investments made by the Imperium, they are often home to one or more Titan Legions, vast walking constructs capable of reigning death to any who stand in their way.