SMS API (Application Program Interface) is nothing but it is a bridge which communicates between the Bulk SMS Gateway and your software or application for having an alert SMS sent automatically as in when it is required.
An SMS API (application programming interface) is a programming language and information on how to access web based panel. An SMS API is mainly used to integrate an APP or a website to directly send messages from the APP or website. To select the best API available in the market I would suggest you go for Serve Enterprise's platform where they have their API available in 11 programming languages like PHP, Java, .Net, Python, Ruby and more. The best part is that they have their API available for free and also have a detailed explanation how to integrate it with an APP or a website.