A jam session is a relatively informal musical event, process, or activity where musicians, typically instrumentalists, play improvised solos and vamp on tunes, songs and chord progressions.
The Philosophers' Cafe is a way of generating conversation where a topic is pre-selected, a person conversant with that topic gives a short opening overview, bringing in as many facets as possible to consider, and the discussion is then opened to the public. A moderator makes sure everyone stays on topic and that the conversation remains respectful of everyone's points of view.
You don't have to be a philosopher or even to have studied the topic to participate. In fact, you can attend and just listen - there is no obligation to speak at all. It is interesting to hear the ensuing debate. It is not a debate in the formal sense of the word, however. There are no winners or losers except that everyone wins by experiencing the Cafe.