It looks to me like a DC->USB adapter... but I can't tell for sure what end is on that from your picture. The current is a little high for USB (USB is usually rated at 1 A, that is rated at 2.1 A).
Looks like it may be some type of DC charger with a 5 volt output like for cell phones or iPods.12 VDC in 5 VDC out.