Doctor of Philosophy, abbreviated Ph.D. or PhD for the Latin Philosophiæ Doctor, meaning "teacher of philosophy", (or, more rarely, D.Phil., for the equivalent Doctor Philosophiæ) is an advanced academic degree awarded by universities. In some, but not all countries in the English-speaking world, it has become the highest degree one can earn (but see also the Higher doctorates awarded by universities in the UK, Ireland and some Commonwealth countries) and applies to graduates in a wide array of disciplines in the sciences and humanities. The Ph.D. has become a requirement for a career as a university professor or researcher in many fields. In addition, many Ph.D. graduates go on to careers in government departments, non-governmental organizations (NGOs), or in the private sector.

Source : wikipedia : http://en.wikipedia.org/wiki/DoctorofPhilosophy