AJAX (Asynchronous JavaScript And XML) and Flash are valuable technologies for creating RIAs(Rich Internet Applications) that mirror each other in overall architecture. Here is a graphical over of how AJAX works.

The differences are found in the implementation of the technology. There are advantages and disadvantages on both sides.

This web page is using AJAX for the content and Flash for presentation. AJAX gets content from a web server searching a database then swaps hidden layers to display alternate content when users click the links to the left.

AJAX is a HTML and JavaScript based browser independent technology. Flash is also browser independent but requires computer users to install an additional program, called the Flash player, to view Flash content.

AJAX is a technology that runs in the browser without installing an additional programs. AJAX uses asynchronous requests to transfer data between the browser and the web server, allowing web pages to request small bits of information from the server instead of refreshing the whole page.

AJAX technology makes Internet applications smaller, faster and more user friendly. AJAX is a technology that uses JavaScript and XML to send and receive data between a web browser and a web server.

The open standards used in AJAX are well defined, and supported by all major browsers. AJAX applications are browser and platform independent. (Cross-Platform, Cross-Browser technology)

AJAX web applications have many benefits over desktop applications, they can reach a larger audience, they are easier to install, easier to update, and easier to develop.

However, Internet applications are not always as "rich" and user-friendly as traditional desktop applications. With AJAX, Internet applications can be made richer (smaller, faster, and easier to use).

AJAX is based on the following standards:

• JavaScript
• XML
• HTML
• CSS

Flash is a Browser Independent Active X Control

You can run Flash without a browser for instance on a CD ROM presentation. But most commonly Flash is found on HTML pages on the internet.

Flash uses a scripting language called ActionScript based on the same specifications as JavaScript. In fact I have often used JavaScript functions I used on HTML web pages in Flash files with only minor modifications to target nested Movie Clips on a timeline instead of web browser DOM elements.

Flash can load data without refreshing using XML similar to AJAX request using the http object . Flash can also use ActionScript commands like LoadVars, Secure XML Socket, and Traditional Gets and Posts commands to load external data.  Both Flash and JavaScript have security restrictions limiting the data they can load to the domain they are hosted or presented by.

Similarities

• Both can use httpXML transfer
• Both use similar scripting languages JavaScript or ActionScript

The Differences

• AJAX can be used without installing an Active X control
• AJAX can be created with notepad for free
• The Flash editor makes beautiful smooth animated transitions
• The Flash editor compiles files so the source cannot be viewed

Wrap-Up

You can use Flash and AJAX together on the same web page!

AJAX is a better then Flash for creating the over all Rich Internet Applications (RIA) making it more assessable to a larger audience, that responds faster and does not require users to download and install an obtrusive plug-in to use your RIA.

Flash should be used like images scattered through out the HTML pages vs. replacing html pages. Flash is better for creating eye candy attention getting animations like website introductions, flashy presentations with Flash effects, software demonstrations with Flash that include audio and animated user interaction, animated cartoons and impressive personal website's.

Flash is good for the WOW factor when it is not mission critical and usability is not a concern, often the case where you simply watch an animation or simulation created in Flash.

Flash can display video and flash captures webcam video feeds. Flash can be combined with  AJAX to provide video conferencing functionality.

The downfalls of Flash are you need to buy the Flash editor from Adobe, formerly Macromedia, and once the .swf file is compiled it's harder to debug and you need the original .FLA source file that created the Flash movie to make any changes.  On the other side of the coin people cannot view the source of your Flash movie so your intellectual property is more secure.