API (application programming interface) is a term you will encounter all the time as a programmer. In my dictionary, an API is an interface for application programming. There are many different kinds. For beginners learning web development, it might be the HTTP API of a web service that might be described as RESTful if it follows certain design conventions. Browsers provide APIs for website to use and third-party JavaScript packages that can be used to provide even more functionality to websites can also be called APIs. The public methods of a class form an API. When you install a Ruby gem and use it in your Rails app, you can think of it as using an API.

An interface can be understood by comparison with an implementation. To use a tool, you need only understand its interface. It is not necessary to consider everything that happens when you use a website like this one because you just need to understand the user interface. Behind the website that you see, there is a huge system of things that are necessary to make the website work, including your web browser, the Internet, and code/content specific to the website that lives on a server somewhere.

An implementation is that internal code that allows a module to carry out that which its interface does.

A web application framework that is the most popular way to develop web applications using the Ruby programming language. The biggest companies with a Ruby on Rails focus are probably Shopify and GitHub.