Whenever an incorrect address is entered into the URL of a web browser an error page is shown. You have probably seen many of them in the past, white pages with Error 404: Not found or along those lines. If you visit a professional website or large named company for example you will see a custom error page. This has the same function of telling the viewer that they have made a mistake and are on the wrong page but with a touch of class. A custom error page looks and feels like your site.