what is the difference between AJAX and DHTML?

can you pls clarify what exactly you mean by focusing on UI?

DHTML is an acronym for Dynamic HTML. Dynamic HTML is a combination of javascript and the document object model that enables you to make a html page react and change to a users actions. This is the technology that makes user interfaces possible, because it lets the user interact with the page. This could inlcude hiding or showing menus or changing one dropdown select based on what was selected in another dropdown select, or even something as complex as gmail, or the new yahoo mail beta.

But, with dhtml, you are limited to using the information already loaded into the browser, and the only way to interact with the server is to refresh the page. In order to interact with the server more quickly and load new information into the current page, ajax is often used. Ajax stands for Asynchronous XML and Javascript. You are able to create a request object inside your current page that loads in XML or often JSON (Javascript Object Notation) which allows you to get things like type ahead suggestions, or load a email in a side pane when someone clicks on it without having to refresh the page.

Most good user interfaces in the next few years will be a combination of DHTML and Ajax. The dhtml will control what happens in the browser and the Ajax will allow dynamic communication with the server so that updates can be made to a database or new information can be loaded without refreshing the page and wasting so much time recreating the page from scratch for every user action.