Bulk SMS API allows developers to integrate SMS capabilities into their applications without needing to log into an SMS portal. The API can be used to send SMS, check message status and delivery reports, and send Unicode messages. To integrate the Bulk SMS API, an application makes an HTTP API call containing parameters like the SMS service URL, user credentials, phone numbers, and message. This triggers the SMS gateway to automatically deliver messages without needing the SMS portal. Sample ASP.NET code is provided to illustrate making an API call to send a test message.