handler are an extension that let you extend the behavior of pages.
For example you could use a
URL Page Handler to change the UI for the page into that of a simple redirect. This will prevent you from having to use the Navigation module for a simple off-site link or top level navigation item.
Another example could be a
GitHub Page Handler that pulls it's content from a github repository to allow collaboration on it. Where all the user has to do is define the source file.
@todo - we need how to build handlers here.