Error pages, also known as error documents or HTTP error responses, are web pages that are displayed to users when a web server encounters an issue while processing a request. These pages inform users that something has gone wrong and typically include an error code, a brief description of the problem, and sometimes suggestions or links to help users navigate back to a functional part of the website.

Common HTTP error codes include:

1. 404 Not Found: This error occurs when the requested resource is not available on the server. It is one of the most well-known error pages and is displayed when a user tries to access a page that doesn’t exist.
2. 500 Internal Server Error: This is a generic error message indicating that the server encountered an unexpected condition that prevented it from fulfilling the request. It doesn’t provide specific details about the nature of the problem.
3. 403 Forbidden: This error indicates that the server understood the request, but it refuses to authorize it. This might be due to insufficient permissions or authentication issues.
4. 401 Unauthorized: This error occurs when access to a resource is restricted, and the user needs proper authentication to access it.
5. 503 Service Unavailable: This error indicates that the server is not ready to handle the request. This might happen during maintenance periods or when the server is temporarily overloaded.

Custom error pages allow website administrators to tailor the messages users see when encountering these errors. Providing clear and helpful information on error pages is important for user experience, as it helps users understand what went wrong and guides them on how to proceed.