A symposium is a type of conference or session that focuses on one particular topic. 
It is often a collection of papers from a number of contributors.  Each contributor will have an opportunity to present and a time for discussion.  This type of session is valuable when multiple perspectives on a particular subject may be of interest to the audience and can be put together in one session.  A symposium is also a time for networking and collaboration with peers and colleagues since these sessions are often comprised of experts in the field who have come together to share ideas.