what is an api
=> On a bare minimum explanation, API are function prototypes which enable the
user to use the functionality of any component by making a function call
without revealing the internal workings of the component.
=> It is a mode of abstraction from the lower layers of software stack , so as
to only provide service when needed for higher layers.
=> An API can manifest in many forms, it can be a simple function call in C,
it can be web service API such as Twitter or FB API, it can be a library call
to OS kernel, or can be an interrupt routine.
=> API can be just a standalone call or in form of libraries. API are part
and parcel of software interaction and data transfer.
Hope this clarifies.