A 404 “Page Not Found” error page looks after expired pages, URLs that are misspelt, cropped by email programs (extremely common) or any other request that comes to the web server that there is no file set to handle.
Having good 404 error handling as a catch-all is recommended, as 404 error pages are almost always amongst the most visited pages on a site.

An excellent 404 error page, and one that we recommend copying, is the Apple 404 error page: www.apple.com/not-real.

This page has the following excellent features:

1. The standard navigation
2. A search box to find stuff on the Apple site.
3. Links to common sections and areas.
4. An explanation of the problem.
5. Ads – it is usually a popular page, so use it to sell!