How to create and use API in ASP.Net

Respected Sir, 

I want breif about API's because now in present I am working as a Trainee software Engineer and here my work is to test API's of Different-different companies.

I need to know that what is API.

What is the use of these API's and how can we create API's 

Please help me related the points because It is important for my position.

Thanks,

Naushad Ansari