It's also a commentary on just what the professor intends to assess.
If it's a book review, that's one thing. No primary sources, so it won't be horribly original, but there's lots of words to choose from in discussing the major arguments offered. The assessment? "Show me that you can read sufficiently to tell me why this book Exists - what value it offers to this field - what arguments it presents, what questions it asks, and so forth. If you're really good, you can review similar works and show me that you understand this book (and author) within the context of this area of the field in question. I'll also assess how well you communicate with others (well, actually, with me) in a written medium."
If it's a short literature review - say, a comparison between two or three brief books or articles, contrasting the major ideas - again, it's not going to be horribly original. It's still secondary sources. The bulk of the work was done by others; the assessment is, "How well does the student read and synthesize information in this field, how well does the student summarize, and how well does the student communicate that knowledge to others in a written medium? Again, if you're really good, you might illuminate a dusty corner, or find an interesting question lying about unanswered - and if this is your major, you could turn that question into Real Research. In an advanced college class for your major, you should crank these out at the rate of 2-3 per course. It's the scholarly equivalent of mining for gold - what you're really looking for is a Good Question To Answer in a Research Paper.
If it's a longer research paper - say, 1500-2500 words using 4-5 articles from a pool of 30-40 articles suggested by the professor - the professor wants to see what you'll make of the information. You're actually doing Research, even if it's strongly directed, and you will likely consult primary sources that these authors used. You'll have to read carefully, find themes - even unintended themes - and make connections between research done by different authors in different contexts for different reasons. You will Make An Argument (e.g. Offer a Thesis). This might be a National History Day paper for a high school student, or even an advanced middle school student. It should be commonplace for college upperclassmen to write at least one of these for each advanced course in a major. The big difference? This is not at all a book report. If you find a complex enough Question to Answer - and you also find that there are Other Sources you could explore in presenting An Answer - you may have found something to use for a Thesis Paper (senior or graduate. I'm feeling generous). (Also, this sort of paper is the currency of trade in graduate school.)
If you find a Good Question, your research may lead to Other Good Questions and Other Interesting Arguments. This is where you transition from Student to Scholar - but you probably won't notice the difference. In your mind, you'll never entirely stop seeing yourself first as a Student - which will bode well for you, as the greatest scholars are first and last voracious learners.
Each discipline has its form of papers - lab reports, musical analysis, art portfolio - but the complexity should be similar.
I suppose I should get back to my questions.