What is the difference between a lecture, a seminar and a tutorial?
They differ in size, and size determines everything else about how they work and what you should do in them.
Lecture. One academic presenting to a large group — dozens to hundreds. The purpose is transmission: covering a body of material, framing a topic, signalling what matters and how the field approaches it. Interaction is minimal by necessity.
What lectures are good for: structure, emphasis, the lecturer's own interpretation, and current material not yet in textbooks. What they are poor for: deep understanding, since passive listening produces weak retention. A lecture is a starting point, not a substitute for reading and practice.
Attend having read something, and note questions rather than transcribing — recordings and slides cover the transcription.
Seminar. A small group — typically 8 to 20 — led by an academic, based on discussion of assigned reading. The purpose is developing and testing your own arguments against others, and learning the discipline's habits of reasoning.
Seminars fail when nobody has done the reading, and there is no way to fake it. Preparation is the entire input, and contributing is part of the assessment in many courses.
Tutorial. The smallest format — one to three students with a tutor. Strongly associated with Oxford and Cambridge, where it is central to teaching and is called a supervision at Cambridge.
The format usually involves submitting written work in advance, then defending and developing it in conversation. There is nowhere to hide, and the individualised challenge is precisely the point. Elsewhere "tutorial" is used more loosely for problem classes or small-group teaching.
Other formats: workshops and labs for practical skills; practicals and fieldwork; and office hours, which are underused and are usually the fastest route to resolving confusion.
Contact hours are a minority of the work — most learning happens independently between them.