The Iron Kingdoms RPG uses a D20 rule system, it is set in the setting of the Iron Kingdoms, the setting where Warmachine and Hordes is all so set.
It is a fantasy style RPG with elements of steam punk.

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The Iron Kingdoms is a campaign setting for the d20 fantasy RPG system and set in the same fictional world as the WARMACHINE miniatures wargame. The Iron Kingdoms setting is created by Privateer Press.One of the key thematic characteristics of the Iron Kingdoms campaign settings is the inclusion of steam, clockwork, and firearm technologies which are commonly absent from many fantasy RPG settings. In the Iron Kingdoms, arcane magic and the refinement of technology arose at roughly the same time, and complement each other. This has set up something of a steampunk infused fantasy game, in a similar way that Shadowrun mixes cyberpunk and fantasy influences.

from wikipedia.com