What are the categories of Ajax frameworks ?

Direct Ajax framework

• Skills needed : HTML, CSS and AJAX
• Pages are developed in HTML and framework deals with HTML elements
• Example : Prototype, XAP, DWR 

Indirect Ajax framework

• Skills needed : HTML and CSS (No need of Ajax or Javascript)
• Code written in High level language (HTML/CSS) is converted to Javascript/Ajax and treated as Direct Ajax framework.
• Example : GWT, RubyJS
• Framework which uses Indirect Ajax framework : Atlas, MagicAjax.NET, ComfortASP.NET, ZumiPage, OutPost

Ajax component framework 

• provide existing components which generates and handles their own HTML
• More rapid development
• Example : Dojo, Yahoo UI, Open Rico, TIBCO, Script.aculo.us

Server side frameworks

• Skills needed : Server side programming
• Ajax calls are used for actions to be done at server side then changes are auto visible at client side.
• Example : GWT, JBossRichFaces, ASP.NET AJAX, Echo2, AJAXPro, ThinWire, ICEFaces, jMaci