Template Messages are pre-approved messages used to initiate conversations or send notifications outside the 24-hour session window. These messages are paid and must be approved by Meta.
Additional information regarding to templates messages are available: Message Template Guidelines
Session (Service) Messages (not supported yet) are free-form messages that can be sent within an active 24-hour conversation window*, which opens when a user replies to your message. These messages are free of charge and do not require pre-approval.
*The 24-hour conversation window is the time period during which you can freely exchange messages with a user after they respond to your WhatsApp message. Each time the user replies, the 24-hour window resets, allowing continued communication without additional costs for message templates. Once the 24-hour period expires, sending any new message requires using a paid Template Message approved by Meta.