For classes, you will enter the main description for the class in the Admin Module when you setup the class. This description is automatically displayed to the clients in the Client Registration Portal for every session of that class you schedule.
The session details are details about this particular session that you can enter when scheduling a class. They show only for that specific scheduled session of the class. For example, if you are skipping a week because of a holiday the session details would be the place to enter that information.