Plenary sessions - everyone comes together usually for a big speech. For example, Steve Jobs talks about new product. Seminars - smaller than plenary, expert(s) talk about a topic. For example, programming for iOSv6 and how it's different than iOS5. This usually is the bulk of a conference. Usually, question and answer at the end. Workshops - doing something or talking about how something is done. Display, showing. Much more conversational than the two above, often has a moderator. For example, troubleshooting common problems in a narrow focus. Companies have conferences to promote internally and externally, to allow for networking, draw inspiration, clear up confusion, large scale training. Usually, an industry or group of companies hosts a conference. Only, the biggest companies would hold their own conference.

I think the answer provided is sufficient enough to explain those words. By the way, do you raise this question after looking up those words on Cambridge Dictionary? :) well...basically all is meeting and to discuss something but there are things distinguish them: 1. conference:- lasting for some days - there are sub topic in a general topic (ex: general topic is world peace, sub topic: war, national military, etc) - or meeting related to business matters 2. plenary session: it is a meeting of a group or an organization (internal meeting) 3. seminar: - speaker:teacher or expert - attended by student of a class or people who meet to study and discuss. 4. workshop: - discuss and/ or perform practical work in a subject/ activity (ex: seminar of how to write a novel)

Hello. Seminars and workshops are like training programs wherein one participates in that said activity to be able to learn or reinforce knowledge and skills of a person. Conferences are similar meetings. In a company, the conduct conferences to confer or discuss business related matters. I hope that I was able to help you. :)