WhatsApp Template Message
WhatsApp Template messages use pre-approved message templates for outbound notifications like delivery alerts and appointment reminders. They can not be used for marketing. You must use a WhatsApp Template Message if more than 24 hours have elapsed since the user last responded to you. Any other type of message will fail.
WhatsApp Session Message
WhatsApp Session messages are any messages sent and received in response to a user-initiated message to your application. A messaging session starts when a user sends your application a message, and lasts for 24 hours from the most recently received message. You can wait for users to message your app, or you can send a Template Message (see left) to invite the user to respond. See below some common examples of how to invite users to message your app.