"Seminar" in my school seems to align with what others have said here: group-discussion based class lead by the professor. Usually, the professor will guide the discussion by having some questions prepared beforehand to ask--usually, the kind of questions that after answering, will keep the students talking for a while. In answering the professor's question, the students usually dialogue with the professor and with each other, giving responses to each others' answers. The professor will give his or her input on the kinds of answers given such that any flaws in the answers (from the perspective of whatever discipline is being taught) will become apparent to the students. Opinions the professor disagrees with are usually allowed provided the student can back it up, though since my school is merely an undergraduate school, such does not occur often; and in the seminar style classes I've been in (though not all of them), to disagree with the professor and back it up would usually require a long tangent that was not related to the discussion at hand anyway.
To prepare, it is good to make sure one has read and have a good understanding of whatever material will be covered during the class period. This may require taking notes on what one has read, depending on one's memory, what material is read, and how one reads for understanding. In the seminar classes I've been in, the material has been too dense for me to read and take notes on usually, so I usually just have had time to read, re-read the trickier sections, skim through to refresh my memory where possible, and then go on memory. Depending on your professor, the discussion topic and questions may or may not be available. If they are available, it is useful to keep them in mind as one reads the material; obviously, if time allows, it would be useful to make sure one has an answer for them all. If they are not available, it is a good idea to keep the main ideas in mind as one reads, note the parts that were confusing or raised questions to onself while reading, and so in a way anticipate the kinds of questions one may be asked. Usually though, one need not worry about such things so long as one actually understood the material one was reading.
During discussion, I usually try to take notes on the discussion at hand--what was discussed, anything important or enlightening revealed during the discussion, any good responses given during discussion (and especially any good responses by the professor, since the professor will usually have a compact or nice way of saying something that the student will take longer to say). This is tricky to do while participating in the discussion at the same time, but it is possible and becomes easier with an increase in class size. Oh, as for participating in the discussion, it is a good idea to try to stay on topic as much as possible in order to keep the discussion from derailing. It is also good to participate as much as possible without hogging the discussion. I usually do this--when I have an answer to questions more quickly than the other students, anyway; sometimes I have to work in order to get in on the discussion--by answering the questions quickly at the beginning of the time, then hesitating before answering later during the class time to see if another student will respond.
In case it is useful to know, my seminar classes have mostly been in philosophy.
Hope that is helpful in some way.
