Ajax is the name of a technology which runs in your browser, not in the server as the php scripts. This way it allows some faster response from the web application. It was introduced in Tikiwiki 2.0, and it has been further included in Tikiwiki 3.0.
In order to use ajax, you need to enable the feature at "Admin > Features > Experimental > Ajax"
When the ajax feature is enabled, it is used in Tiki at:
- registration time, verifying the password strengh and if both copies of the password written match or not: Registration
- when opening a wiki page for edition Edit
- the Module minichat
- the Shoutjax module, which is like Shoutbox but ajax'ified.
- and others.