Frequently on the Internet, you would have come across registration forms where selection of certain fields cause the entire page to refresh in order to show you the details specific to that selection.
An example of that would be countries and cities. It's a natural thing for a web developer to want a person to input the country and city with a select box instead of an input box. This is because cities and countries rarely change, and you really don't want your users making up countries or cities!
However, there are many thousands of cities when you consider the number of countries in the world, and to print them all in a simple registration form is extremely impractical, not to mention their sheer number, as we will see in a later chapter!
Earlier the developers would have come up with a solution like: to add an event watcher, wait for the Country select box to be changed, and when this happened, the entire "state" of the form would be sent to the server and a new page...