Let me back up and give you a little history. When someone comes to your site’s webserver and asks for a page that doesn’t exist, like http://www.mattcutts.com/asdfasdfasdf , most web servers are configured to return an HTTP status code of 404, which means that the page was “Not Found.” If someone links to a page on your site that doesn’t exist, most webservers give a pretty sucky experience: visitors usually land on a pretty useless page, and search engines might not give you full credit for those 404 errors.

t would be trivial to mail some of these people and say “Hey, I noticed you linked to my site (thank you!) but the link is broken, so users will get a 404 page. Would you mind changing your link on page A to point to the right page, which is url B?” When the other site fixes their link, their visitors find your site directly, plus all search engines can follow those links and give you credit for them.