A Ph.D., or Doctor of Philosophy, is a high-level degree earned after a period of three or more years of graduate-level study, culminating in the creation, submission, presentation and defense of a research dissertation.
For some professions, such as university professor or researcher, the Ph.D. is pretty much de rigor. Most Ph.D.s are earned as a means of contributing original research findings to an academic community, field of study or professional discipline.