Why are some courses described as "seminars"? I noticed that occasionally,
the title of a course will be something like "Special Seminar in Topic X".
A student can take this special seminar course for 3 course credits,
just like a regular course.
What is the difference between a seminar course and a regular course,
and why is the seminar course referred to as a "seminar"?

(I do have some knowledge about what a seminar course is,
but I am asking this question
because I feel that the answer may be useful for others.)
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.