It is the third level of university education. The first level (1st Degree) is called undergraduate. The second degree you do as a specialization to your first degree is called a Masters or post- Graduate. When you what to become an expert in your filed of study you do a third degree called a Dr of Philosophy, hence the abbreviation PhD you could call it- Philosophy Doctor. This is the highest level of education in one field. But once can have multiple fields of study if they so wish and have the time and strength to do it.
Funny footnote: PhD can also be in a joke situation be called Permanent head Damage!!