WhatsApp Session Messages
A session is the 24-hours period from the time a user contacts your business.
Messages that you send to and receive from users within this period are outbound and inbound session messages respectively.
They are usually conversational messages, and so do not need to follow a template or be approved by WhatsApp beforehand.
The content of the messages can be of a marketing nature if desired.
WhatsApp Template Messages
These messages are typically used to deliver notification content, such as a booking confirmation or a delivery alert.
They can be used to reach out to the customer outside of a session, or even during one, if your business process requires it.
These messages must be approved by WhatsApp before you can use them, and templates for marketing messages are not allowed.