I reported on the change from Beta2 of Visual Studio 2005 to the July CTP in a previous post. According to latest information from Microsoft, the interface will undergo another change for RTM. Here it is from the horse's mouth:
Callbacks enable ASP.NET controls to request data from the server using client script instead of posting back the page. For example, the TreeView control uses callbacks to populate child nodes when the parent node is expanded; this avoids the need to render the complete data hierarchy to the client on the initial page request.
Controls will typically retrieve data from a...