What are the various designations/stages in the academic career of the person
Like EngergyNumbers said, you should tailor your classification to the culture of your targeted audience. This list of academic ranks for various countries might be helpful for that.

Also, "Scientist" and "Researcher" doesn't really mean anything. A postdoc, assistant professor, principal investigator, or someone doing research in the commercial sector can be a scientist or a researcher.

I think you know this but just in case: "postdoc" is an (often) temporary job that people take up after obtaining a PhD, and not a qualification. Some people do not do postdocs, and go directly into teaching, industry or become assistant professors after getting their PhD.

Finally, "Post Grad" can refer to both masters degree or PhD students. It is common to say "applying to graduate/grad school" to when referring to applying to a PhD course.