If you have a 12volt standard electrical system in your vehicle.
It is both positive and negative. The travel inverter converts this 12 volt DC current into 110v 60hz AC current so you can plug in your electronics that require standard household current to charge/operate.
There is a built in USB port if you device will accept this for 12 volt power. It also has the power oultet with ground for 110v 6hz power.
Link below has a good picture and description. Hope this helps!

http://www.sears.com/diehard-120-watt-ultra-slim-travel-inverter-with/p-02871520000P