API is an interface that defines how and by what means software or a computer program requests data and processes from other subroutines and operating systems. API asks in HTTP and the responder answers in XML or JSON. These back and forth qustions and answers are the platform that allows for interactive sharing and collaboration of information and ideas in the Web 2.0 world.
AII - Application Interaction Interface
AII is an interaction interface that is built on top of an API, often called a web application. Interaction interfaces increase the value of APIs because they add features and functionality that are sometimes lacking in the original API.
Mashups result from combining two or more separate APIs. This results in a distinct web service completely different from the original applications' functionality.