The Furnace is a Vanilla item. It can smelt raw ores or dust. In order to smelt, it requires some form of fuel, like Coal. When smelting, it produces light and has a fiery particle effect. You can smelt many things, like Iron Ore.
Generator
Furnace Cart
Iron Furnace
Tunnel Bore
Dark Matter Furnace
Energy Collector
Lava Fabricator
TEKKIT Tutorial Furnaces Read desc. for more information