They are small, discussion based classes that have more specific of a topic than most other courses. Participation is usually worth more. Specific assignments depend on the professor, but there is usually a presentation, historiography paper, and a research assignment.
If you'd like to know anything else feel free to ask
I have taken four history seminars. Also, if you want to get more of an idea of what different professors expect, there are many seminars' syllabi available to download on the history department's site.
