query with wiring to outside light?

i have an outside halogen light with a pir sensor that doesn`t work, i want to replace it with a new one with two lights and one sensor trouble is there are two wires going into the light but my neighbour has the exact same light with only one wire going in, why has mine got two and his only one, if you can explain in simple terms please, this would be of great assistance to me, Thanks in advance if you can help.

GEORGE
October 2011