The reason relatively inexperienced web designers might be a bit surprised by our example is that their view of how the web works is somewhat restricted to techniques that have been around since Sir Tim Berners-Lee invented all this technology years ago. That is, the web is a client-server system. The client is your computer and its browser. It sends a request in HTTP (HyperText Transfer Protocol) format to a server computer. This happens when you click on a link or type an address into the browser's address box. The specified address is routed to the proper web server, which retrieves a page and sends it back to the browser as a response. The web server may simply load the page from disk and send it out, or it might run some code first to modify it. Either way, an HTML (HyperText Markup Language) document is delivered to the client and displayed by the browser. This request-response mechanism is fundamental to the way the web operates.
This page that was delivered by the web server may contain forms with elements (like drop-down lists), but the most basic way the web operates (or used to) is that the form is filled in completely by the user, and then submitted when the user clicks on a submit button. At this point, another HTTP request is sent to the server (usually, but not necessarily the same server) containing the form information. The form is processed by the server, and the result is that a new HTML document is delivered to the client browser. Again, the response is an HTML web page to be loaded.
Clearly, something quite different is going on in our example, because doing something with the form on the client side (selecting the brand of automobile) changed the form without reloading the page from the server. How this works is worth understanding, since it has become basic to the operation of the web in today's world.
And, just as important, it allows us to create web applications that function in much the same way as desktop applications. They just use a browser for user interaction, and often store their data on the web. Thus, the applications and their data don't need to be on your computer (since they are on the web) and you can run them from any computer, anyplace in the world so long as you have web access. By eliminating the need to reload a page on every change, operation is fast and smooth enough to enable us to build useable applications.
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