seminar: It is a regular (e.g. weekly or monthly etc.) meeting of the faculty and students of a department who discuss and participate, where one or more faculty members give their talk on a particular topic.
Conference: It is gathering of students, faculty, experts from different fields other than academics who generally meet e.g. annually at a particular place in the country or abroad, where several scholars, both invited and other participants present their recent findings on several areas of in a branch or an interdisciplinary area. It will also include poster presentation and guest lecture series from an expert group as well. It can one to week duration at a grand scale.
Symposium: It is academic gathering where participants discuss about a topic. It is primarily include an equal opportunity to discuss and present about the topic by the audience and/or the invited faculty memeber or an expert.
Workshop: It is primarily a teaching or training about the development in a research area, where invited faculty or expert gives taliks and/or teach techniques (methodology or lab techniques) to the novice researchers, primarily young scholars and teachers.