why two wires to my outside light when neighbours only has one?

I have an outside halogen light with a PIR sensor i want to move the light as it dosn`t work and replace it with a double light with PIR sensor but there are two wires going in to the light, my neighbour has the exact same light but with only one wire going in, can somebody please exsplain this to me why in simple terms, Thanks in advance if you can help.

George Aylesbury.
October 2011