Why are some courses described as "seminars"?
It depends on where you are located. In many english-speaking countries the word 'seminar' refers to a research talk.

In german universities a seminar is a particular type of course in which students are assigned some topic on which they have to prepare a report or a presentation or both (usually). Almost universally one aim of such a course is to let students develop their presentation skills and to give them an opportunity to engage with the literature.