A session is a single item on an event agenda: a keynote, a breakout, a workshop, a panel, a poster session or anything else with its own time slot.
Session is the umbrella term for any agenda item. Keynote, breakout, workshop, panel, roundtable and poster session are all sessions with a format attached. Using the umbrella term in your system and the format in your copy keeps an agenda readable without inventing a new object type for every shape of room.
A time, a room, a format, a capacity, an owner, and a decision about who can attend. The last one is where agendas break. A session anyone can walk into and a session restricted by ticket type behave differently at the door, and that has to be settled before the agenda is published rather than patched on the day.
A track groups sessions by topic. Concurrency decides which run at the same time. A session belongs to one track and competes with whatever is scheduled against it, which is why putting two sessions from the same track head to head is the most common agenda mistake there is.
Sessions carry their own type, capacity, track and access rules, can be uploaded in bulk by CSV, and can be made private where a track is invite-only. Admins and speakers can still enter a session that has reached its capacity. See event agenda builder.