* Graduate studies is a broader term encompassing *all* studies undertaken after completing a bachelor's degree. This includes master's degrees, doctoral degrees, professional degrees (like law, medicine, or MBA), and postgraduate diplomas or certificates.
* Postgraduate studies specifically refers to studies undertaken *after* a graduate degree. This usually means further education beyond a master's degree, typically leading to a doctoral degree or a specialized postgraduate certificate/diploma. Sometimes it can refer to further professional development courses taken after an initial professional qualification.
In short: All postgraduate studies are graduate studies, but not all graduate studies are postgraduate studies. Graduate studies is the umbrella term; postgraduate studies is a more specific subset within it.