What is the differrence between Unit Testing and API Testing ?

Unit Testing is an activity to verify functionality of smallest part of compilable code.

API Testing, Application Programming Interface Testing, is mainly about testing subroutines in a particular application. 

So if we take these definitions, at times we as tester may refer both of these terms interchangeably. This will happen when we consider Unit Testing  as an activity for testing Functions of C++ and API testing will again be same thing is this case.

Ideally API Testing is something which is done by Testing team on an application build, while Unit testing is done by Developers and can be done without actually building the application. 

We need to research more on it from practical point of view and will update this article again. Please feel free to share your thoughts through comments.