Section 14.3 How Web Applications Work
Most of the time, when your browser requests a file from a web server, the server simply transmits the contents of the file back to the browser. But sometimes, the “file” your browser requests isn't really a file at all.
Try typing the following URL into your browser:
You'll get back a page of search results about Microsoft from the Google search engine (at least, you will unless Google has changed how it performs searches since this chapter was written). How did this happen?
Well, your browser did what it always does when you type in a URL:
The browser opened a network connection to the web server named google.com
The browser requested the “file” named /search?q=Microsoft from the web server
What the web server did at this point is different than the example above. There's no “file” named “/search?q=Microsoft” on the Google web server. Instead, the web server ran a web application to search through Google's massive database of websites for pages that mention “Microsoft”. The web application dynamically generated an HTML document containing the search results, and the web server transmitted that document back to the browser.
The browser rendered the HTML document
As far as your browser is concerned, there is no difference between requesting a “static” HTML file from a web server, and requesting a dynamically generated HTML file. It's up to the web server to examine the request submitted by the web browser to determine whether it should serve up a regular document, or run a web application to generate a response.
Anytime you're browsing the web, and you notice that the URL of the page you're viewing has a question mark (?), you can be fairly certain that the page was generated “on the fly” by a web application on a web server. By the way, the portion of the URL that comes after the ? is called the “query string,” and contains input for the web application. Try changing the query string by substituting “Firefox” for “Microsoft” to see what I mean.
In summary, a (server-side) web application is a program that is run by a web server to produce output in response to an incoming request from a web browser.