public speaking是公开演讲的意思,而conversation则是对话或者交谈的意思,区别可以从以下例子看出:

If you find someone's mistake, try to have conversation in private, not in public speaking, then will be easy to be accepted 如果你发现某人的错误,尽量私下交谈,而不是公开演讲,这样比较容易被接受。