The Difference Between a Bachelor's, a Master's and a PhD Degree
If you watch too much television (guilty!), you may notice the ridiculous number of attractive, perky, totally-not-exhausted twentysomethings on police procedurals and scifi shows who have six PhDs in the Sciences or some obscure language (for the record, as a former Classical Languages major, I do not consider either Latin or Ancient Greek to be "obscure." Yes, I am a nerd. Yes, I parenthetically footnote myself a lot).

This accumulation of higher degrees, of course, does not happen. And certainly not by the time you are 25. 

When I'm asked what a PhD (a Doctorate in Philosophy) is about, I'm reminded of my head doctoral supervisor Hugh Kennedy's advice. We were talking about doing other degrees (I had previously done an MA and an MLIS, which is what's known as a "professional" Master's, on top of the Classics BA) and I asked him if it was worth it to do more than one PhD. 

His advice? "Unless you're doing something very different, such as switching over from the Humanities to the Sciences, no. What a PhD teaches you are research skills. These can apply to any subject you choose to research and you can use them in any job you get from now on."

In a Bachelor's degree, you learn basic research skills, such as locating published sources in libraries and online, evaluating them,  and using them to support an argument. You also learn how to cite your sources properly. In effect, you learn how to write essays. If you do a high-level (300 or 400) class, you learn how to write very long (10,000 words or more) essays, including a final one known as a "dissertation."

In a Master's degree, you still learn how to argue a position, but your thesis statement is more pointed and your source approach is far deeper and more complex. The research here is known as a "literature review." You learn how to locate published and unpublished sources across your topic, do a complete analysis of them, and then argue a position. In business, the final product would be known as a "white paper." You finish up your Master's degree (unless it's a professional degree, which is more hands-on) with a "thesis" (still known as a "dissertation" in the UK) that is usually about 15,000 words long. You must also defend your thesis in an oral presentation that is like a lecture.

In a PhD, the goal is also to do a literature review but as a step to the ultimate goal of doing original research. You are not just taking existing, already-examined sources and arguing a thesis. You are (ideally) collecting previously unexplored sources and collating information in a new way that advances your field. Your dissertation (U.S.)/thesis (UK) is generally 80,000-100,000 words long and can be converted into a "monograph" (academic book) after its successful written and oral defense.

This system, which was formed in Western European universities of the Middle Ages, is based on the medieval guild system of Apprentice (Bachelor's), Journeyman (Master's), and Master (PhD/Doctorate). An Apprentice learns the trade. A Journeyman is an expert in the trade. A Master is an expert at the top of his (women were rarely allowed in medieval guilds) field who has his own business and has completed a "masterwork." In the university system, your masterwork is a PhD dissertation (U.S.) or thesis (UK).

So, if you need someone to do research for you, it's usually best to go with someone who has a Master's or PhD. The exception is someone, like a local historian or a master in the trades, who has spent many years studying their particular subject at a practical level. Professionals, like Medical Doctors (MD) or lawyers (J.D., LL.M., and S.J.D.), are another exception.

But that is a whole other discussion.