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Those days everybody is using Web 2.0 and AJAX, but everyone understand a different meaning of those technologies. Here I'll explain my interpretation.
In short I'll use this formula:
Web 2.0 = XHTML Strict + CSS2 + AJAX
It means the end of using html tables for positioning objects in the page (HTML 4.0 or XHTML Transactional). Web 2.0 uses CSS2 properties to do it. In this formula AJAX is the XMLHttpRequest JavaScript object, that allows to get HTTP data using JavaScript code (and prevents reloading the whole page).
A good reference for more of those technologies is W3Schools. In addition there is a very good presentation for best practices (and accessibility) called Unobstructive AJAX.