Answer 1

I searched on Google and found nothing specific, so I think you're talking about Web APIs in general, not Android specific.

Given the statement above, I'd say that a Web API is an interface that provide data to several clients.

The client request data from the server, that returns a file in a specific format (JSON, XML, YAML, etc).

The client may be an Android application, a web application from a browser, a server, a script, a batch script, etc.

An example would be an e-commerce Android application that requests a list of products from the server and lists it for user visualization.