thats right, postgrad is a term to mean any qualification that requires a bachelors degree (ie post graduation). there are some business ones that take work experience but they are the exception. i dont think honours is generally considered postgrad.
phd are generally for research. you cannot get a phd without contributing something new to the pool of human knowledge, which generally implies research. most research positions (like at the csiro etc) require a phd. this is why i say it is for research. as it was explained to me, a phd is like your apprenticeship for life as a researcher, where you learn how to conduct valid research.
things like a masters, postgraduate diplomas/certificates are extra qualifications akin to an undergrad degree on steriods. they are done for a variety of reasons as discussed in this thread.
collectively these things are referred to as postgraduate study. most postgrad information days are targeted at people wanting to do masters. from what i have seen people tend to use masters degrees as a stepping stone. they may have worked for five or so years and their career has plateaued so they are not really happy where they are or looking for new challenges. a masters, be it in the field they studied as an undergrad or a different field altogether, can provide them with more interesting and varied options to continue their career.
there are no particularly hard and fast rules, requirements will vary from uni to uni, course to course. for example you can do a masters by research which can lead to a phd. in general a masters by coursework will not allow you to get into a phd, but this varies from field to field. i am pretty sure its true for science, but i know a guy who has an MBA and is thinking about doing a DBA (he also has no undergrad qualification, he became an accountant before you needed a degree to be one
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