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Ajax (Asynchronous JavaScript and XML) is a method of building interactive applications for the Web that process user requests immediately. Ajax combines several programming tools including JavaScript, dynamic HTML (DHTML), Extensible Markup Language (XML), cascading style sheets (CSS), the Document Object Model (DOM), and the Microsoft object, XMLHttpRequest. Ajax allows content on Web pages to update immediately when a user performs an action, unlike an HTTP request, during which users must wait for a whole new page to load. For example, a weather forecasting site could display local conditions on one side of the page without delay after a user types in a zip code.

Google Maps is one well-known application that uses Ajax. The interface allows the user to change views and manipulate the map in real time. Ajax applications do not require installation of a plug-in, but work directly with a Web browser. Because of the technique’s reliance on XMLHttpRequest, early applications worked only with Microsoft’s Internet Explorer browser, but most other browsers now support Ajax.

Applications created with Ajax use an engine that acts as an intermediary between a user’s browser and the server from which it is requesting information. Instead of loading a traditional Web page, the user’s browser loads the Ajax engine, which displays the page the user sees. The engine continues to run in the background, using JavaScript to communicate with the Web browser. User input or clicking on the page sends a JavaScript call to the Ajax engine, which can respond instantly in many cases. If the engine needs additional data, it requests it from the server, usually using XML, while it is simultaneously updating the page.

Ajax is not a proprietary technology or a packaged product. Web developers have been using JavaScript and XML in combination for several years. Jesse James Garrett of the consultancy firm Adaptive Path is credited with coining the name “Ajax” as a shorthand way to refer to the specific technologies involved in a current approach.

The ten best tutorials, and examples. 

1. Ajax Tutorial: Ajax What is it Good For?
   1. This site covers not only how to code in Ajax, and to implement it into your site, but also the history of it, and how it came to be.  It is a good place to start.
2. Rasmus’ 30 second Ajax Tutorial
   1. This is a quick and dirty introduction to Ajax.  It doesn’t go in depth, but in 30 seconds (ok, a few minutes) you should be able to comprehend it; if you have coding knowledge.
3. Ajax: Getting Started
   1. This is from the Mozilla developer center.  This is also a somewhat basic introduction to Ajax, but it is quite nicely laid out.
4. Interactivity with Remote Scripting
   1. This is just like the first two, basic, yet understandable.  A good read if you are still having trouble understanding Ajax, and remote scripting.
5. Ajax Examples
   1. This site isn’t so much a tutorial, as it is a comprehensive list of Ajax examples.  The author lists real world uses of Ajax to give you ideas on what you can use it for in your sites.
6. Ajax (programming)
   1. Wikipedia usually has a plethora of information and links for many, many topics.  This is also true for Ajax programming.  There is some great information here, and make sure to check out the external links at the end of the wiki.
7. Ajax in ASP.NET
   1. If you are an ASP.NET developer, than this is the place to take a look at once you understand the basics of Ajax.  Microsoft is busy working on their own implementation of Ajax; called Atlas, but it isn’t ready yet.
8. Ajax with J2EE
   1. Another tutorial of Ajax, this time implementing with J2EE.
9. Ruby on Rails
   1. There is a very active group of developers that swear by Ruby on Rails.  It is a framework that accelerates the development of Ajax.  It is considered a very fleshed out and easy to use version of Ajax.  As a bonus; Ruby is incredibly easy to learn to code in.
10. Ajax: A New Approach to Web Applications
    1. It’s probably best to actually include the article that coined the term Ajax.  It doesn’t have code examples, but does give a nice walkthrough of how Ajax actually works.