The main responsibility of a Hibernate SessionFactory is to create and manage Sessions.
The main responsibility of a Hibernate Session is to provide a CRUD interface for mapped classes, and also access to the more versitile Criteria API.
Really, it's more difficult to find commonalities than it is to find differences between these two classes...
Have a look at the API documentation.
Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.