For language subjects, what happens during a seminar?
It probably differs from subject to subject. That being said, I'll describe what happened in my German 5 seminars this semester.
For me, they were a weekly, 2-hour class of about 20-25 people (about what you'd have in a school classroom). They are typically quite interactive or involve some form of discussion between the class or at least tables.
In the first hour of each class, we'd work through culturally-related texts and try to gain an understanding of concepts specific to the German culture. This involves watching videos, reading texts in German, and discussing what particular ideas meant to us in German. Then, in the 2nd hour of the class, we'd focus on language learning. This was typically learning a new grammar rule and then doing a couple of practice drills relating to it. Unlike the tutorials, the seminars were basically unrelated to lecture content and were independent. There wasn't much expected work to be brought in, but some optional homework tasks and vocab lists were set for each seminar.