The main difference between public speaking and conversation is: public speaking is more formal. public speaking is more structured or organized. public speaking requires some kind of event or platform. all of the above.

The main difference between public speaking and conversation is:

All of the above.

Explanation:

Public speaking is no doubt a formal affair. One can't just climb up a stage and use informal and low profile words to address people.

Public speaking is more structured and or organized, one has to properly prepare or rehearse before climbing up to stage and addressing  the audience, he/she must always be aware of what they are speaking and to what extent is it right.

One can't deny the fact that public speaking requires some kind of event or platform. When we are speaking to public, it means there is an  appreciable amount of audience we are catering to and it directly corresponds to an event.