HRZ is correct, but in my experience with zoning stuff, a setback refers to how close you can construct your improvement (building).
A buffer is something you have to do between abutting property uses, typically you have to keep the area clear, unpaved, planted with trees, etc...
For example, if you're going to put a light commerical next to a residential, you might have a 25' setback on the building, but they may want a planted 10' buffer somewhere between your building and the residential property abutting.