From http://www.w3schools.com (Copyright Refsnes Data)

AJAX Examples

These examples demonstrate JavaScript used together with XML (AJAX).

Examples Using the XMLHttpRequest Object

Load a textfile into an HTML element with XML HTTP
How to use an XMLHttpRequest to retrieve new content in an HTML element.

Load an XML file with XML HTTP
How to send an XMLHttpRequest to retrieve data when the user clicks a button.

Make a HEAD request with XML HTTP
How to send an XMLHttpRequest to retrieve HTML header data.

Make a specified HEAD request with XML HTTP
How to send an XMLHttpRequest to retrieve a specific part of the HTML header data.

Display an XML file as an HTML table
How to display an XML file as an HTML table

Using XMLHttp when user types in an input field: