A custom error page is a feature of most Web server software that allows you to replace default error messages with ones you create. The default error messages tend to be fairly generic, and not particularly user-friendly, so making custom messages for a site is recommended. You can make them look more like the rest of a site, and/or provide better recovery navigation. The Not Found (404) error is the one users are most likely to encounter, so it is the most likely to be customized.