What's the difference between a reorder buffer and an instruction window? In an out-of-order processor, what is the difference between a reorder buffer and an instruction window?  

Wikipedia says:
"In particular, in a conventional design, the instruction window consists of all instructions which are in the Reorder Buffer (ROB)."  

Does this mean that the instruction window isn't a seperate hardware structure?
Correct.  The instruction window is not a hardware structure.  It is the set of instructions that is contained in the reorder buffer at any moment in time.  Looked at another way: the reorder buffer is a container (like a drinking cup is a container) and the instruction window is the thing the reorder buffer contains (like milk, juice or water is the thing contained in a drinking cup.)