There is no difference whatsoever.

both of them have exactly the same meaning. "someone" and "somebody" mean "有人"or "某人". for instance There's somebody(someone) on the telephone for you .有人打电话给你。 somebody also means "大人物，有头有脸的人物"。 like He thinks himself somebody .他以名人自居。 Someone is used when indicating that there is ONE. Thus it is used mainly to show alone. Somebody is used to indicate a human being. Just a body. Used mainly to emphasize a presence, not a number. P.S. Somebody比Someone较常用于口语.语气更强烈。