* PhD student: This is a broader term encompassing anyone enrolled in a doctoral program. It includes those in the early stages of their studies, still taking coursework and qualifying exams.
* PhD candidate: This term generally refers to a PhD student who has completed all coursework and qualifying exams, and has officially begun their dissertation research. They are *candidates* for the PhD degree because they have met all the requirements *except* the completion and defense of their dissertation.
In short, all PhD candidates are PhD students, but not all PhD students are PhD candidates. The transition from student to candidate marks a significant milestone in the doctoral journey.