A PhD is a university degree that a student gets after several years of advanced study in a subject. Once you have a PhD, you can be called "Dr. So-and-so."
The abbreviation PhD stands for "Doctor of Philosophy," and it's also called a Doctorate. Students who attend graduate school and continue their studies beyond a Master's Degree eventually receive a PhD. Most college professors have a PhD. The name of the degree comes from the Latin phrase philosophiae doctor, and the "philosophy" part of the name comes from the Greek word philosophia, "love of wisdom."