The difference between lectures and seminars is that in the first instance at any rate the lecturer does most of the work. You are encouraged to ask questions, but you are not required to have read up the subject before the event. For seminars, by contrast, you must have done the reading beforehand. The responsibility for a good lecture lies with the lecturer, the responsibility for a good seminar lies with the you, the students. The first three seminars are, however, different, in that they are of an introductory character and will more resemble lectures.
Tutorials are small-group discussions.
At the postgraduate level the rule is that the group must not exceed five. Written work plays an important part at the college level. Some tutors will treat it as obligatory, others will give you the opportunity if you want to take it of having your written work looked at. You should take that opportunity. Oxford is one of the few places in which you can still get written work read and discussed.