Hi,
I'll try: A formal meeting are very much official. That means: has the following characteristics: It is set in advanced, in a specific place, at a specific time. There is a chairman. Minutes are taken, sumarrised and send to all attendees (+ to all who should know). Have usually action items.... Informal meeting are just the opposite: Not official, normally not set in the calendar, no minutes, no records, no chairman, no summary, can take place any time and any where (during dinner, even at the toilets...)..... can be very short (1 minute 5, 10 or much more). Informal meetings are very important to any living organization (don't under estimate it's importance !)..... much of the communication in the organization takes place in that way.
Best regards,