Minecraft wiki entry for Hopper, associated text: Furnace
Blast Furnace
Smoker
A working hopper pointing into top of a furnace deposits only into the ingredient slot. It can push any item, including items that can't be smelted by the furnace. A hopper pointing into the side of a furnace deposits into the fuel slot, and only items that are usable as fuel. A hopper below a furnace pulls everything from the output slot and empty buckets from the fuel slot left over from using lava buckets as fuel. When a hopper removes items from a furnace, the experience points are 'stored' in the furnace until a player removes at least one smelted item, or the furnace block is broken.