oral (platform) presentation - 10 minute talk with powerpoint slides (10-20) in front of an audience followed by a 5 min q&a. has a moderator/discussant to lead the discussion.
poster presentation (oral) - usually 2-3 minutes with several powerpoint slides (2-4) along with a poster that is put up in a room with other oral posters, usually in the same site/topic. the moderator leads the discussion. not sure how long the q&a period is, but i think its about 5 min.
poster - put a poster together, and stand by it for usually a 4 hour period of time. may (not) get questions asked by people walking / reading them.