PhD stands for Doctor of Philosophy (Yes, it seems odd how it is the wrong way round, but it is taken from the latin – doctor philosophiae). It is another degree you can do after you have completed your first degree, and doesn’t have to be in philosophy! Philosophy is used in a more general way here, meaning love of wisdom. Some PhD’s are are taught and some are research only – like mine!
Hi kamilo12, Tess’ answer is really good, and I’ll just add a little about what it typically involves. This is usually a series of scientific studies on a particular theme. It often asks for a researcher to first identify what isn’t known in a particular area, and then conduct a study or series of studies designed to examine those questions.