When working with Ajax you can choose to render you GUI on the client. This means you need a “thick” javascript client with lots of logic. But it makes the Ajax calls to the server small and quick.
The other option is to make the server create and render the GUI and stream the pre-chewd HTML to the client. This makes for thin clients with little javascript and is often easier for developers that are more confertable in managed C#/VB.NET code (who isn’t). But it makes the ajax call much larger and slower thus removing some of the benefits of Ajax (imho).
Brad Neuberg has a good post about this in his Blog.