Before learning how to write a web application, you need to understand a bit about how web browsers work, and how web applications interact with users.
The World-Wide Web is composed of thousands of web servers connected to the Internet. Each web server contains lots of different kinds of files that web browsers can request: HTML pages, image files, audio files, and other resources. When you click a link on a web page, the web browser sends a request to the web server, which transmits the requested file back to the browser.
Once the web browser has downloaded the requested file, it needs to do something with it. Web browsers know how to render an HTML document, show images, play audio files, and so on. If the web browser doesn’t know what to do with a file, it usually prompts the user to save the file, so the user can do something with it.
A URL (“Uniform Resource Locator”) is the address of a resource on the Web. It has three sections: the protocol (ex. https:) the browser uses to request the resource, the server where the document is located (ex. docs.python.org), and the path to the requested resource on the server (ex. /3/library/index.html).
If you want to see the file transmitted by the web server to the browser, right-click in the browser window and choose View Page Source (your browser’s option to view the source may be slightly different). The browser shows you the file it downloaded from the web server.
