Here's how the web works:
- A user types a web address (or clicks a link) into their browser.
- The browser asks a server for the page corresponding to that web address.
- The server responds with data, generally text encoded as HTML.
- The browser displays this HTML to the user.
To build a web application we don't need to worry about the browser (someone else has built that) so we just need to concern ourselves with instructing the server what to respond with. All of Ruby on Rails is concerned with step 3.