Ajax, shorthand for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes.Entire page need not be reloaded every time a request is sent.

Ajax is not a technology in itself, it is a group of technology

• The Ajax technique uses XMLHttpRequest (Javascript) object to exchange data with the web server.
• Any format including XML , JSON and sometimes even plain text are used for transferring data between the server and client. These files may be created dynamically by some form of server-side scripting (Java / PHP).
• Dom allows to access and update the content, structure and style of documents using Javascript.
• XHTML (or HTML) , CSS and helps in marking up and styling information.

As like any other technique, AJAX too has pros and cons.Am listing a couple of them here.

Pros :

• One great advantage that anyone will never deny is the speed and the resposiveness.
• As the Ajax requests are Asynchronous, user can send simultaneous requests and still get response for all his requests.

Cons :

• Users generally expect that clicking the back button in web applications will move their browser to the last page it loaded, and in Ajax that not might be the case. Though there are various technical solution to this problem using hidden IFRAMEs, I would suggest the best solution is a proper design which doesn’t tempt the user to click “Back button”.
• Difficulty of the user to bookmark a particular state of the application, due to the dynamic update of the content.
• As a developer, annoying fact is the browser differences.Browser differences sucks.

While building next generation Web Application (Web 2.0), I believe rather than doing some workarounds for these disadvantages, it is better to leave them as such and concentrate more on the design.