Whenever you visit a web page on the Internet, your computer (the client), will request data from a server, through the HTTP protocol. This happens even before the page that has been requested, is displayed in the browser.
This answer is sent through the HTTP head, that will serve the status code, which provides information about the status of the request. Normal webpages that do respond correctly, returns an http status 200 OK. The statuscode 200 is not seen, as the webpage will load after the statuscode has been received, however the exact opposite happens, when the header returns a 404 Page Not Found.
HTTP 404 or 404 not found is a http-standard response code. This response code means that a client, could communicate with a server, but that the server could not return, what was initially requested by the client.
The standard response, will be that the hosting server is generating a “404 Not Found” web-page, when the user attempts to reach a page, through a broken or dead link. The 404 error-code is probably the most recognized HTTP-status code on the Internet.
The problems with 404-error pages
Bad user experience
From a user perspective reaching a 404-error page, while browsing a webpage or entering a webpage from a search engine, is a very bad user experience.
Bad user experiences can hurt your reputation and hurt the users will to revisit your webpage. Also, it can stop them from reaching their desired destinations of your website and potentially lead them to another website, in search of the same information.
Loss in Search Engine Ranking
Search Engines, especially Google, sees 404-error pages as a lack of quality, the same way as a user might do. Too many error pages can potentially lead to an algorithmic search engine penalty. Google has several quality-filters and penalties, that amongst other, look at the number of error-pages on a website. Google updates like Google Panda, Fred and Phantom is notorious known, too see this technical issue, as a sign of low quality. Getting hit by one of these penalties, can severely hurt your search engine visibility.
External links to a website is votes of confidence from a search engine perspective. However, if external links are pointing to pages on a website, that does not exist anymore, the valuable link-equity will be lost and therefore, not assist the webpage in better rankings in the search engines.
How to deal with 404-error pages
First and foremost, it is crucial to do regularly scans of your entire website, to find and identify 404-error pages throughout your website.
All of these pages, has to be 301 redirected into other pages, eliminating the error-page and reestablishing both the user experience and the link-equity from external and internal links. This will reestablish the lost value, for both users and search engines.
Making better web experiences and increasing search engine rankings.