If users of your app can purchase tangible goods through it, then the app must get information about the availability of such products. One way to accomplish this is through a web service. A web service is an app (web app or web process) that runs on a web server and to which clients (desktop or mobile apps) can connect to access resources such as an inventory, product images, orders, and payments. The most web-centric way to access such resources is through the use of RESTful API. Representational State Transfer (REST) provides a way for a computer to communicate with other computers, akin to the way people browse the web.
When you visit a web page, the page will likely offer options (links) that you can use to access content that is related to this page or the workflow (transaction) in which you are engaged. You then analyze each link to decide which related page to go to next. The RESTful API provides clients with an organic way to interact...