Should I use Ajax?

Ajax becomes necessary when you want a dynamic site or pages, and that they be updated by loading data from the server, after the display, at the request of the user, or to renew the content.

The asynchronous mode and XMLHttpRequest

Ajax, in addition to the components of Dynamic HTML, including JavaScript, CSS, DOM, has the asynchronous mode that is provided by the XMLHttpRequest object.

• The interaction with the server is done without reloading the whole page, without waiting for user actions, the results fit to the page in a fluid way once they are available.
• XHR can pass commands to the server and send data to it.
• The combination of DHTML and XMLHttpRequest thus produce pages that operate in an interactive way like conventional software, and Ajax is so the basis of Web applications and of the development of RIA (Rich Internet Application).
• We can send data from a form and have them checked by a script on the server in conjunction with a database. Then be asked for more information if needed.
• Loading images while the user reads the contents of the page.

Ajax or iframes?

They help to integrate a content from other pages. But this content is not part of the current page, it can not directly be accessed from it and modified. Iframes are perfect to view additional content from the same site or another site.
An example of iframe is given by the CSS Tab Panel that allows to display pages or images selected by the user.
The contents are displayed in an bounded area while Ajax can change any element of the page itself.