Aluminum is an internal enhancement metal that, when burned, clears out all the metals inside the allomancer, including itself. It was revealed during Vin's imprisonment in Kredik Shaw by the Inquisitors questioning her.
It is paired with it's alloy, duralumin, which burns all of the active metals in an intense flash. Aluminium is not affected by steelpushes or ironpulls. An easy way to use this to your advantage would be to use aluminium jewelry, and to make weapons and bullets out of aluminium.
A Misting that burns Aluminum is known as an Aluminum Gnat, due to the uselessness of the power on its own.