The essential purpose of an error page is to inform the visitor about the problem or error in accessing your website. There are several types of errors and most of them have their distinct error codes, the common ones are.
301: It means that the requested page has been permanently moved
404: Bad URL, unable to found the page
401: The access to the particular file or directory is restricted for the user
500: It returns as Internal Server Error, and mostly happens when some script is not functioning properly.
The server provides the basic error pages, while the custom error pages can be created for the valid HTTP status.