Traditional web navigation model is based on web pages: clicking on a link (pointing to a another page) takes you to another page (url) and the browser refreshes the whole page. Most of the time, the new content will replace the previous content in the same browser window (or tab). I label this navigation model as "between-page web navigation/action".
An alternative model is "within-page based navigation/action" where the navigation or user activity remains in the same interface container. For example, changing the month of a calendar only refreshes that calendar division, not the whole page. Browser plug-ins can implement this model, such as Flash or Silverlight (see TargetScope and SilveOS). Another way is to use scripts, or AJAX, which is gaining popularity recently.
One good practice is to implement MDI (multiple div(ision) interface), similar to the MDI (multiple document interface) or TDI (tabbed document interface) in desktop applications. One of the advantages is it allows multi-tasking and multi-threaded navigation. It is certainly useful for many web applications; Yahoo's new email is a good example.