It is called the Foundation exam, and as such covers background information that all the other tests in the series (TS:70-526, TS:70-528, etc.) will expect you to have learnt already.

In other words, even though it is arguably the hardest of the TS tests to take, you should study for it, pass it, and get it out of the way first.

It is harder than the the other tests because its horizontal, versus vertical -- you have to know a little about quite a few sections in the Framework (strike that -- you have to know quite a bit about quite a few sections), and unless you jump straight to memorizing questions via Pass4Sure or other test preparations methods, studying for this exam will take quite a lot of time.

What it Means to Employers
For employers, the TS:70-536 test is probably more important that the other TS tests: what's tested here is general computing knowledge -- albeit expressed in Framework terms --which one would find in most languages or frameworks (ie, C, C++, Java, etc.), such as ValueTypes, Reference Types, Stacks, Heaps,Threads, Mutexes, etc..
Of course, since we are dealing with a Microsoft product, in addition to these cross-platform concepts, it also tests .NET Framework specific concepts such as Installation, CAS, RBS, etc. that you won't automatically find elsewhere.

Comparing 70-526 to other TS Exams
The other tests, 70-526 (WinForms) and 70-528 (WebForms), etc. delve more into specific technologies that Microsoft sell, rather than computer concepts. 
For example, in the 70-526 exam, you'll find questions about how to use the BackgroundWorker component rather than the underlying System.Thread classes.  That said, just because most of the questions are about these upper level classes and components, doesn't mean you won't see a question or two about the underlying classes and theories. For example, even though threading questions in 70-526 might focus on the BackgroundWorker component, you can expect to see something about Thread.Start/Stop/Join as well.

How to Study for the TS:70-536 Foundation Exam
The question really comes down to whether you want to know something -- or just pass an exam.

...How to Pass at all Costs:
If you just want to pass the test, and are much less interested in actually knowing the Framework, thinking that anything you may really need is only a couple of web browser searches away, then stop reading. Really. 
Go somewhere else, and download brain dumps of the questions, memorize them, pas your test.

...And How to Study for it Properly:
On the other hand, if you want to learn the foundations of the Framework, and in the process, study for certification, then I strongly recommend the official book (the cover is shown above), which you will find on amazon or any other computer book outlet.
The primary benefit of the book is that you will go over in detail all the parts of the Framework you will be covered in the exam.
 With copious notes, lots of coffee, and watching out for small paragraphs of information just about where you're eyes will glaze over (they always seem to choose questions from the obscure, rather than the glaring), you'll get to know the Framework.

..Although watch out:
My only problem with the book is that although it covers in detail the Framework, it doesn't prepare you for the examination. 
Let me explain. Yes it has great chapters, and yes it has test questions at the end of each chapter. 
But the questions are worded in such a way as to test your ability to comprehend what was taught in each chapter, not prepare you for the exam. The book's questions are too...direct, too obvious, and not as ...devious(?) ...as the real certification examination.
The real certification questions are more tricky, almost trying to catch you out, rather than asking you straight forward questions expecting straight forward answers.
For this reason, I recommend that the you use the above book to get a strong understanding of the material, but then pop over to StepupTransform (link is listed at the bottom of the page) for a final prep evening before the exam. What they offer is a set of questions that are very similar to what you will find in the exam (but not as a blatent a direct rip as, eg, Pass4Sure) with the right choice expressed in very clear code examples.