PhD is an abbreviation for "Doctor of Philosophy" (Philosophiae Doctor).
The reason for the name is historical. Most universities used to have only four majors: divinity, law, medicine, and philosophy. All other modern subjects (such as physics, English, math) were part of philosophy, and so most subjects still use PhD as their doctoral degree.
To receive a PhD you first must receive a BA or BS degree (4 year undergraduate degree). You then apply for a Master's degree and then apply to be accepted to a PhD program. Some PhD programs combine the Master's and PhD into one.