Benjawi.

A 404 error is is vital to your site if done well. By not specifically telling your web server how to handle 404 errors, you end up with a very plain and generic error page if you click on a dead link or type in the page name wrong. This will just say something like "Not Found - The requested URL was not found on this server." on a plain white page. Not only is this unhelpful, it's also a road block for your users. Although it's an error page, it should still look like your website. Static sites can achieve this through the use of a .htaccess file, but if you are using a CMS this can be handled for you.

The thing that you need to remember is that no one wants to find themselves on this error page. Some of the most common reasons that people find themselves on a 404 page are:

1. An internal link that is broken
2. A search-engine link that is out of date
3. A mistyped URL
4. An out of date bookmark/favourite

Most of the time, if you end up on one of these pages, you're not going to be happy. So if user of your site ends up on a 404 page of your site, this is a golden opportunity to apologise, even if it is their fault for landing on your 404 page. You don't want to just point the blame away from yourself, after all, it might not even be their fault. Try and use phrases that don't entirely push blame away from yourself such as "possibly" or "you could have" instead of just "you have". Just because they've ended up on this page doesn't mean you don't have what they were looking for.

You need to consider things such as the page that a user bookmarked just doesn't exist anymore, so why not try and help them find the page that they were trying to go to or a page of similar relevance. Most cases, having a search function will help. Links to the homepage, the archives pages or your sitemap could be helpful. Try not to use too much jargon either. If I told my nan that she'd landed on a 404 page she'd have no idea what I was on about even though she uses the Internet every day and loves it. A simple "page not found" is more meaningful to users than "error 404". 

You can even go a step further by having a contact form on your error page allowing your users to explain what they were looking for but now can't find. Although you don't see this very often, it's not a bad method to use as a side to your error message. This is not only good for users, but good for you as you can learn about the problem and fix it to prevent this happening again and you could also send the user the correct link that they were looking for instead of making them look elsewhere for the content that they want.