Also referred to as routines, procedures refer to the specific tasks or functions a program performs. For example, Twitter provides a search API for developers to explore data for analytical purposes.
The protocol is the format used to communicate data between applications. This can get complicated, though. Applications may not rely on the same format. But, more on this later.
Think of tools as a set of building blocks – the components needed to construct new programs.
APIs are needed to bring applications together in order to perform a designed function built around sharing data and executing pre-defined processes. They work as the middle man, allowing developers to build new programmatic interactions between the various applications people and businesses use on a daily basis.
Breaking down the API definition can help build a better understanding of just how this type of interface works.