Post-session Templates also are known as Notifications and can be sent anytime even after 24 hours of the user's last message. These templates have to be approved by WhatsApp before usage and thus need to follow WhatsApp Regulations. 


You can create and access your post-session templates in Nurture tab 


Characteristics of Post-session Templates

NatureAny message which is sent after 24 hrs from the user's last message.
Initiate ConversationOnly messages of transactional nature are allowed. Also, only the message templates which are pre-approved by the WhatsApp team can be sent.
Initiate ConversationYou can initiate a conversation with a new user but only after getting his consent
Content TypesYou can send rich-text (bold, italics, strike, monospace) messages.
PersonalizationPossible using dynamic parameters
There is a limit on total messages you can send per day. The limit starts from 1000 msgs/day can be increased to 100,000 msgs/day. 
Cost WhatsApp charges brand for these messages.

You can also send media (Image/file) in templates now.

If you want to know about the different types of messages (Session and Post session), click here here

You can check out Example templates here.