The terms doctorate and Ph.D. are usually used interchangeably. Though they often mean the same thing, not always. A Ph.D. is a form of doctorate, but the difference is a little more complicated than the simple analogy that “everyone from Boston is from Massachusetts, but not everyone from Massachusetts is from Boston.”
Ph.D. stands for Doctor of Philosophy, and it is the highest academic honor bestowed in the U.S. It often involves years of postgraduate study on a single subject and concentration, starting with course work, then possible internships and teaching positions, and finally ending with a dissertation writing and subsequent defense. In addition, the U.S. Department of Education recognizes more than 20 degree titles that have an equivalent status to the Ph.D. (a list can be downloaded here). What all of these have in common is the idea that the holder of such a title is more than a practitioner of a given subject or career field, but also a scholar with the ability to teach and advise others in an official capacity. Professors at universities are almost always required to hold a Ph.D.
This differs from say an M.D. or J.D. (medical doctor and juris doctor), where the holder is bestowed the ability to work in a professional capacity, but not in a scholarly one. To use the lawyer example, a J.D. is someone who attended three years of law school after obtaining a four-year B.A. or B.S. This person has the ability to practice law in the state or states he or she is certified in, but does not have the background to teach law at a university, or write in scholarly journals on a specific legal subject, like constitutional law. His or her law school education did not require the passing of specific exams to be able to move to the dissertation process, writing a dissertation, nor the subsequent defense of the dissertation.
In fact, a Ph.D. in law requires one to not only have a J.D., but also an LLM, or Master of Laws, before he or she can even enroll in the program. What that means is, yes, a Ph.D. is a form of a doctorate, but so is a basic law degree, and as you can see, one requires much more course work, and as such, is much more prestigious. If you watch a political talk show and see someone with a title like “professor of constitutional law at George Washington University,” that person has likely attained the highest scholarly level in the field of constitutional law, a Ph.D., and that is two notches above the average practicing lawyer with a three-year J.D.
The best way to look at it is that a Ph.D. or its equivalent is the highest academic honor bestowed in the United States. It is called a doctorate, but so are many professional degrees, like a law degree or medical degree, which are also prestigious and require a lot of work, just not as much as the Ph.D. does.