A lecture is a simple event to announce a talk.
Features: poster creation, participant management, ...
A meeting is an event that defines an agenda with multiple talks.
Features: timetable, minutes, poster creation, participants management, ...
A conference is a complex event with features to manage the whole life cycle of a conference.
Features: call for abstracts, registration, e-payment, timetable, badges creation, paper reviewing,...
Check also here for group events: https://www.aisociety-unipd.it/