This is for the United States. A Master's takes two years of full time study, and involves coursework plus the writing of a thesis. A thesis will be relatively short, maybe around fifty pages, and will constitute a piece of original research in a field. A PhD takes significantly longer, at a minimum probably five or six years, and as many as ten or more depending on various factors, including subfield (some subfields require extensive language study, for instance). In the United States the PhD will also involve coursework, usually two or three years' worth, followed by a period of individual study in preparation for comprehensive exams which are designed to ensure a candidate's mastery of their major field of study plus additional subfields of concentration. After advancing to candidacy by passing their comprehensive exams, a PhD-seeker will assemble a committee of three to five established scholars (this number depends somewhat on individual program) to oversee their dissertation, which is a book-length original study on a chosen topic and involves significant research using archival and other primary sources. For reference, my own dissertation was rather short, and it checks in at over 300 pages. Anyway, once they have written the dissertation, they and their committee will meet for a formal "defense" of the work, during which the established scholars will discuss the work's contributions to the field and suggest ways that the newly-minted PhD might revise and adapt the piece in the future. If all goes well the committee will approve the dissertation, and the candidate will earn the PhD.

Many people earn an MA in the course of getting a PhD - it is a not insignificant accomplishment, but a PhD qualifies a scholar as an expert in his chosen field in the eyes of her peers.