What's the difference between a reorder buffer and an instruction window?
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.)