The ajax framework is specifically for doing connections between a client browser and a server, but isn't really a framework in and of itself. It's a combination of javascript, (x)html DOM models, and xml sockets to make changes to a webpage without haveing to do a full "refresh". It doesn't really run on the client or server side but is more a means of interaction between the two. I'd recommend reading up at wikipedia on exactly how the technology works http://en.wikipedia.org/wiki/Ajax_(programming)

This is kind of like answering the question, "Which is better; a sedan or a flatbed truck?" It has to depend on what you mainly want to do and your point of view. Any framework is a collection of common routines or functions that saves the programmer the time of writing them over and over again every time one needs them. Different frameworks are biased in favor of different functionalities, but all AJAX frameworks have in common the code to handle the JavaScript XMLHttpRequest object. So by implication, AJAX frameworks are client-side because JavaScript does not run server-side. However there are AJAX frameworks written in languages that run server-side (PHP, Python, et al.). I believe these frameworks output JavaScript code, but I am not confident this is absolutely true. The advantage to this is that if one knows the underlying language, one can write code to generate dynamic HTML pages that have AJAX functionality. The disadvantage is that one is committed to using that language exclusively. If you found a nice piece of code written in a different server-side language, you'd have to rewrite it in your language of choice so you could still use your server-side framework. If you used a client-side AJAX framework, it wouldn't matter what language was being run server-side. Which is better? Impossible to say. Some think server-side languages should be selected in terms of their strengths. Some think in terms of uniformity and are more confident when all the server-side code is written in one language. What you think will provide the correct answer for you.