AJAX stands for Asynchronous JavaScript with XML( AJAX ). But what does it mean?
In simple English, it means that developers will be able to use AJAX to build more responsive web sites. What this means for DotNetNuke is that you may begin to see fewer reloads on an entire page and that if something changes on a page, the AJAX functionality will redraw only the part of the page that changes. As a result, interactive pages may begin to respond as screens do on desktop software. In Shaun Walker's blog entry, he refers to "partial rendering capabilities for more efficient and responsive application usability" of a page. Microsoft refers to this as "enhancing the user experience."
In terms of DotNetNuke, Walker's blog describes in very general detail how DotNetNuke is supporting AJAX, including support even if the host site hasn't installed the AJAX framework.
There's also a detailed description of implementing AJAX in a module -- but this discussion is probably more useful for a module developer than a user.
For most DNN users, I think, what we'll see is increasing use of AJAX in the core product, as well as in modules. We're in the early stages, too, so don't expect to see a lot of AJAX functionality right off of the bat.
Mitchel Sellers has been testing AJAX features with the core DNN modules and has documented some of the results in
his blog. Some modules support "partial rendering" (this is a setting that's buried in the module control's configuration. Mitchel describes some modules for which this is useful, and also modules for which this isn't useful.
Long term, I think that most of us will see that advantages of AJAX as the DNN teams adds it to the core of DNN and as module developers AJAX features to their modules. If you are a module developer, you'll want to consider this for your modules.