HTTP Status Codes

1xx: Informational Messages

Purely for informational purposes

200 OK - request was successful, body contains requested content
202 Accepted - request was processed successfully, but may not contain the requested body (for example with async processing)
204 No Content - No contentent available
205 Reset Content - Indicates, that the client should reset the document view
206 Partial Content - The body only contains a part of the content. The other headers specify more.

301 Moved Permanently - Resource is at new URL
303 See Other - The resource is temporarily at a new URL. The new URL is passed in the Location header.
304 Not Modified - The resource has not changed and the client should use the cached version. (This can be determined by the ETag sent by the client - a hash of the page)

400 Bad Request: the request was malformed.
401 Unauthorized: request requires authentication. The client can repeat the request with the Authorization header. If the client already included the Authorization header, then the credentials were wrong.
403 Forbidden: server has denied access to the resource.
404 Not Found: indicates that the resource is invalid and does not exist on the server. The other codes in this class include:
405 Method Not Allowed: invalid HTTP verb used in the request line, or the server does not support that verb.
409 Conflict: the server could not complete the request because the client is trying to modify a resource that is newer than the client's timestamp. Conflicts arise mostly for PUT requests during collaborative edits on a resource.

500 Internal Server Error: the server had some sort of crash or internal error that stopped it from fulfilling the request.
501 Not Implemented: the server does not yet support the requested functionality.
503 Service Unavailable: this could happen if an internal system on the server has failed or the server is overloaded. Typically, the server won't even respond and the request will time out.