The undergraduate seminar paper

For the first time in a long time, I have the opportunity to teach a bona fide seminar. The "seminar paper" is a pretty ubiquitous feature of that experience. But to my surprise, I've never thought very explicitly about what an undergraduate seminar paper is supposed to be. So I'm interested in how all of you have explained this to your students: What's its main rhetorical function? What's the proposed length? What are the main components?

Thanks!