Conversation is like a web wizard. It is a series of web pages inside a http session. It is smaller than a http session. It is especially useful in multiple browser tabs, where all tabs belongs to the same http session.
Process scope lives longer than the http session. It is a business process saved in the database. So, it can survive server reboots. And it can have multiple actors each having its own http session.
Conversation scope falls between the request scope and session scope.
Normally the request scoped objects do not survive a redirect.
As opposed to this, Conversation scope maintains the objects even thorugh redirect.
If you get a chance to pick up the book, there are a number of illustrations in chapter 8, 9, and 10 describing conversation scope in-depth (from basic conversations, to concurrent conversations, and nest conversations). The process scope is also discussed in-depth in chapter 24.