SNMP INFORM is similar to Traps but is more reliable.
When an Agent sends a Trap to the Network Management System (Manager), the Manager receives the Trap and no acknowledgement is sent to Agent. By this way, the Agent doesn’t know whether the Trap has been received by Manager or not. Whereas in Inform Request, the Manager upon receiving the Inform Request can acknowledge the Agent with a SNMP Response PDU. By this way, Agent can be able to know whether Inform has reached or not.
If the Agent didn’t receive the Acknowledgement from the Manager, it can resend the Inform Request again.