I'm not currently doing a PhD, but I have worked in research, so I may as well answer. In England, a postdoc is a research project undertaken by a student who has recently obtained a PhD. The post-doc will involve research and publishing, but without a viva having to be written, or any of the associated "coursework" that a PhD student does. A post-doc may be involved with aiding PhD students in the lab, and also may have a certain amount of teaching to do if the postdoc is undertaken at a university. Postdoc contracts generally seem to last 2-4 years, and a student may complete between one and three of them before becoming a fully fledged lecturer. You don't get qualifications as such when you do a postdoc - it's more something to put on your CV, along with the puslications, which seem to be more frequent that when you do a PhD.
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