Master of Science

In the United States, the Master of Science degree (MS) typically requires a thesis, although some technical or clinical programs (as well as programs in mathematics may require an exam or an internship instead.

Sometimes the MS is a terminal degree, meaning that it's the highest degree available in the field (although beleaguered students in the midst of exams will often claim that terminal degrees are those that kill you before you finish...). Students in the aforementioned technical and clinical programs--audiologists, engineers, computer scientists, speech therapists, and so on--generally stop after earning the MS unless they plan to become professors, in which case the PhD provides the necessary calcification.

Usually, though, the MS serves as a step on the way to the PhD. Weirdly enough, different programs use the MS in opposite ways: some award it to students who pass their prelims, while others use it as a consolation prize for those students who bomb their prelims and thus fail out of graduate school. This (as you might expect) leads to lots'n'lots of confusion. My program used the former method, and after I passed my prelims and got my master's, I bounced around the campus for a while jokingly trying (and understandably failing) to get people to call me "Master Leighton". A friend of mine in a biology program--one that uses the latter method--saw me across the quad and seriously considered calling the men in white coats; she thought I'd failed out and slipped into some sort of manic hysterical counter-reaction. Fortunately she asked me about it first, which saved me from having to spend an otherwise happy day in a mental asylum somewhere....

Once again, things differ in the UK. Here it is possible to gain an MSc by dint of either
1. Having a degree in the field you are doing your MSc in. This degree will typically be taught, and then be assessed by examination or thesis.
2. Having a degree in something else. You undertake a two year taught course, which is assessed by examination. This is rather like a BSc course, but without the first year.
Of course, your mileage may vary. The degree typically awarded as a consolation prize for not having a very good PhD thesis is the MPhil. Note that PhD's in the UK are not examined, and indeed not taught at all. There is at least one taught Doctorate in the UK, the University of Southampton's EngD in Transport Engineering, but this is very much the exception.