I would imagine the primary difference between formal and informal meetings would be how structured they are. For example, a weekly team meeting with a pre-determined agenda, set time and location, and meeting minutes recorded would be a formal meeting, whereas a quick ad hoc conference call between project members to clarify something that was previously discussed might be considered informal. I don't know that there is any hard and fast definition, though, nor do I recall any focus on this distinction on the exam or any prep materials I encountered.
From page 295 of the PMBOK Guide (5th Ed): "Although causal discussions may be construed as a meeting, most projectmeetings are more formal with a prearranged time, place, and agenda." This seems to suggest that both types are possible (though formal is more common).