[edited by: encyclo at 6:03 pm (utc) on Nov. 17, 2006]
[edit reason] switched to example.com [/edit]
A simple example is if you use server-side includes for dynamic content. Your included script can display today's date, file contents, etc. But there's no way for the requesting agent to know that this happened.
Here's a good article written by Tim Berners-Lee many years ago about this topic: [w3.org...]