PhD candidate vs PhD student I see researchers working on their PhD calling themselves PhD Candidates, other call themselves PhD students. What's the difference between a PhD Candidate and a PhD Student? 

According to two posters on the WordReferences forums:


  This terms vary from university to university, usually a PhD student is granted the candidate status after completion of a “comprehensive examination”, which occurs sometime after the first year.


and


  Here in the U.S., a “Ph.D. Candidate” is a student who has completed all of the academic requirements for their degree, except their dissertation.


So this works in the USA and Canada I presume, because they have structured PhD programs where you attend courses as part of your PhD. Unlike most of the universities in Europe where a PhD student starts off directly with his/her PhD thesis. (Europe does have structured programs as well, but they are not as widely spread yet).
In some countries (e.g. The Netherlands where I obtained my PhD degree) you are not considered a student but a paid employee (staff) with the university. To discern between these, people sometimes translate their status to English using term "PhD candidate". Btw, this has nothing to do with the length of the program or your progress.