A roundtable discussion is a small-group session where participants talk as peers around a shared topic rather than listening to a presentation.
A panel has an audience. A roundtable does not. Everyone in the room is expected to contribute, which is why the format caps out at roughly twelve people before it quietly turns into a panel with bad sightlines.
Roundtables fail in two ways: silence, or one person talking for forty minutes. Both are facilitation problems. A good facilitator prepares three opening questions, names people directly rather than asking the room, and is willing to interrupt. Brief them properly and give them the attendee list in advance.
Peers. Mixing a junior coordinator with three VPs produces a session where one person says nothing. Many events sort roundtables by seniority, by company size, or by the specific problem attendees said they were bringing, which is a good use of a registration question.
Roundtables need capacity caps and sign-up, because walk-in numbers destroy the format. They also need rooms that are actually round, or at least square, since theater seating defeats the purpose before anyone speaks. See event agenda builder.