There are two types of sms,
1. Promotional sms
2. Transactional sms.
Promotional sms is used for business promotion or any other advertising purpose. As per TRAI rule, Promotional sms is only for business / product promotion. Messages can not be sent on DND enabled mobile numbers. Message sending time: 9 AM to 9 PM.
Transactional sms is used for any type of transactional notification, for example: You can send sms to all your students to alert them for payment due date. After payment, you can send sms for payment confirmation at their mobiles. Also you can send any notification for examination and holidays.