Error codes that are commonly seen by users are the 400 and 500 series error codes. These error codes are generated by the web server itself. Not all web server error codes are explained in this document. However, all error codes are listed.

400 series error codes are used when the client or user has caused some kind of problem when engaging the web server.

400 - Bad Request
Usually indicates a syntax error of some sort (a space in the URL for example) in the requested URL.

401 - Authorization Required
Shown when a user has not entered server required authentication.

402 - Payment Required
Requested information cannot be accessed without payment.

403 - Forbidden
Read privileges of requested file or directory aren’t set so that you can view the requested information. Commonly seen when directory browsing is disabled.

404 - Not Found
Requested file does not exist. This is the most commonly seen error and typically it is due to a URL syntax error or the file being deleted.

405 - Method Not Allowed

406 - Not Acceptable
The web server is unable to generate a client allowed response.

407 - Proxy Authentication Required
Client must authenticate with proxy server.

408 - Request Timeout
The client did not respond within limits set by the web server.

409 - Conflict

410 - Gone
Requested information is no longer available and no forwarding address is available. Typically a 404 error will be shown but a 410 error can be used to show users that the information is permanently unavailable.

411 - Length Required

412 - Precondition Failed

413 - Request Entity Too Large

414 - Request-URI Too Long
Server refuses to provide requested information because user submitted a URL that exceeds the limit of the server.

415 - Unsupported Media Type

416 - Requested Range Not Satisfiable
When a client sends acceptable file size parameters to the server and the user requests a file that does not meet these parameters.

417 - Expectation Failed

500 series error codes are displayed to the user when the web server itself is having an issue providing the user with the requested information.

500 - Internal Server Error
Usually indicates a server configuration error. User request can not be fulfilled until server configuration is corrected.

501 - Not Implemented
The web server does not support the requested feature or service.

502 - Bad Gateway
Only occurs when a web server is acting as a proxy or gateway. Indicates an invalid response from the next server in line.

503 - Service Unavailable
Web server can not process request at that moment due to overloading or maintenance. Usually a temporary condition.

504 - Gateway Timeout
Only occurs when a web server is acting as a proxy or gateway. Indicates that the next server in line did not respond in a timely fashion.

505 - HTTP Version Not Supported
Server does not or refuses to support the HTTP version that was used by the client to request information.

For a more detailed technical explanation of all web server error codes take a look at this excerpt from RFC 2616 (Chapter 10 - Status Code Definitions).