This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Your post is not exactly clear but I think U want to know if U need to know everything about XML to do Implement Ajax in your application?
No you dont need to know a lot about XML, all you need to know is that you can represent data using XML Tags. You will also need to know how to generate the XML response on the server side. Thats about all you need to know to get started and do some decent work.