These codes are called variously:
Browser return codes; ( Web ) Server return/result/error codes; HTML error codes This is an attempt to gather them all into one place in an easy to reference format. A brief anecdotal comment follows each error code, click the error code number for the official definition of each code. If this is not sufficient, refer to rfc2616.

100 Continue
The client SHOULD continue with its request.
101 Switching Protocols

200 OK
Transmission was successful; no error; OK
201 Created
Your request has generated a response located @ ( URL attached to the error code ).
202 Accepted
Your request has been accepted and is now being processed.
203 Non-Authoritative Information
At least some of the information transmitted came from a source other than the server you are connected to.
204 No Content
The server processed your request, but there was nothing to transmit.
205 Reset Content
206 Partial Content
The transmission was only part of the URL ( usually by request of the browser ).

300 Multiple Choices
The requested information exists in several locations... and the server does not know which one to send.
This can normally be resolved by making the request with a more specific URL.
301 Moved Permanently
The resource has been moved. Sometimes the new location will follow the error code.
302 Found
The resource has been moved. The move is temporary, and the browser will probably handle this error with
out any help from the operator.
303 See Other
The resource resides on another server. The URL generally follows the error code.
304 Not Modified
The client has requested a resource ( web page ) that has previously been sent in this secession.
The server has checked to see if the page/resource has been updated, and it has not.
This code tells the browser to use the copy the browser has in it's cache.
305 Use Proxy
306 Reserved
307 Temporary Redirect

400 Bad Request
The server could not understand the request. This is usually because the request string was garbled
or the browser is using a request methodology the server does not understand.
401 Unauthorized
The requesting machine and/or user is not authorized. Invalid username and/or password.
402 Payment Required
Payment required.
403 Forbidden
Access denied ( for some reason other than authentication failure ) such as file permissions.
404 Not Found
The server could not find the requested URL.
405 Method Not Allowed
The request method utilized by your browser ( for this URL ) is not allowed.
406 Not Acceptable
The server has located the requested resource ( web page ) but it does not meet the requirements
specified in the request header.
407 Proxy Authentication Required
Proxy authentication required.
408 Request Time-out
Request time out. The server has not received a request in the max wait time set by the administrator.
409 Conflict
There is a conflict for the state of the resource. ( locked for update; in the process of being deleted etc.)
410 Gone
The resource has been removed and no forwarding URL or replacement resource is available.
411 Length Required
Access denied. The credentials provided were accepted, but are not sufficient to provide access to the resource.
412 Precondition Failed
Precondition failed
413 Request Entity Too Large
The browser's request would result in a response that exceeds the byte limit set by the server administrator.
414 Request-URI Too Large
Request URI is too large.
415 Unsupported Media Type
Unsupported or invalid media type.

416 Requested range not satisfiable
417 Expectation Failed

500 Internal Server Error
Internal server error. There are many things that can cause this, most of them are the responsibility
of the server administrator. ( file permissions, bad cgi code, illegal input not properly filtered )
501 Not Implemented
Request method ( one of: GET, POST, HEAD, PUT, DELETE, CONNECT, TRACE ) not implemented on this server.
502 Bad Gateway
The server received an error from an upstream server. ( The server you requested from needs information
from another machine to complete the output to your browser.... and this "other machine" has erred. )
503 Service Unavailable
Service temporarily unavailable, usually due to maintenance, max clients already logged on, or connection
overloaded ( packet loss ).
504 Gateway Time-out
The server received no response from an upstream server. ( similar to 502 )
505 HTTP Version not supported
HTTP version not supported.

8181
Authentication certificate has expired.