* Postgraduate is a broad term encompassing *all* studies undertaken after completing a bachelor's degree. This includes master's degrees, doctoral degrees (PhDs, EdDs, etc.), postgraduate diplomas, and postgraduate certificates.
* Master's degree is a *specific type* of postgraduate qualification. It's a higher-level degree that typically builds upon the knowledge gained in a bachelor's degree.
Think of it like this: all squares are rectangles, but not all rectangles are squares. All master's degrees are postgraduate studies, but not all postgraduate studies are master's degrees.