I’m using the term webapp on my own, in phpnuke they’re called modules. I’m not sure of the capabilities of a plugin or widget yet.
As an example:
Last year for Christmas I wrote a php based app for a family christmas exchange where people could post their list of items they wanted/needed. There were a couple of php files: one for viewing the list of names, one for viewing the list of stuff if you clicked on a name, and one for editing your list.
So I guess my question is could a plugin or widget do something like that? And if it can, can it temporarily take the main focus spot on the site (this probably shows my ignorance of wordpress)