“404” is one of those bits of computer code geekery which will be recognised by even the non-IT-savvy internet user. But as a website owner, it’s very important that you correctly serve up 404s – indicating that someone’s on the right domain, but the page they’ve asked for doesn’t exist. An excellent and very thorough introductory post on the Google Webmaster Central blog called Do 404s hurt my site? is a good place to brush up on what a “404” should really be used for. If your website is managed for you by someone else, I’d forward the article to them and get them to confirm that non-existent pages are being handled correctly.