The purpose of a Forge is to make unfired steel from iron. The Forge itself contains two shallow bins for storing and organising your resources and a 'forge' with a small two-by-two slot on top. To convert the Iron you have to drop the iron (smelted) into the hole. This can be qquite difficult. Once you have converted the iron, it will become Unfired Steel and require being 'fired' to become usable in any recipes as steel.
This recipe follows the style of other useful buildings.