A formal meeting is a pre-planned event where two or more people come together to discuss specific matters for the purposes of achieving a specific goal. An informal meeting is more casual, and less planning is involved.
Formal meetings have a formal agenda, but many informal meetings do not. Unlike formal meetings, participants are free to speak their minds in informal meetings.
Informal meetings have no formal requirements, while formal meetings are conducted by a chairperson and follow an agenda.
Informal meetings can take place anywhere, such as a restaurant or a coffee shop, while a formal meeting takes place at a pre-planned place at a particular time.