Question

The main difference between public speaking and conversation is:

Answer 1

Public speaking is more formal

Public speaking is more structured or organized.

Public speaking requires some kind of event or platform.

Answer 2

The main difference between public speaking and conversation is SIZE OF THE AUDIENCE.
The size of audience for public speaking is usually bigger than that of conversation, conversation usually involve very few people. 
The other differences are these: for public speaking, the tone is formal, the speaking is more sturctured and the delivery mode [posture and loudness of voice] is formal.