Online HTTP Header

HTTP = Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML hypertext pages. Development of HTTP was coordinated by the W3C (World Wide Web Consortium) and the IETF (Internet Engineering Task Force), culminating in the publication of a series of RFCs, most notably RFC 2616 (1999), which defines HTTP/1.1, the version of HTTP in common use today.
Source: WikiPedia
You can test ports other than port 80 by specifying the port behind the url, like: www.google.com:88

HTTP response codes:

100 Continue
The server has not rejected the initial part of the request, and the client should continue.
101 Switching Protocols
The server agrees with the client's request to change protocols.
200 OK 
The request has succeeded.
201 Created
The request has been fulfilled and resulted in a new resource being created.
202 Accepted
The request has been accepted for processing, but the processing has not been completed.
203 Non-Authoritative Information
The returned headers are not definitive, but are gathered from a local or a third-party copy.
204 No Content
The server has fulfilled the request but does not need to return any output.
205 Reset Content
The server has fulfilled the request and the client should reset the document view (primarily intended to make data-entry easier).
206 Partial Content
The server has fulfilled the partial GET request for the resource.
300 Multiple Choices 
There are several resources that match this request, and the client was redirected to one of them.
301 Moved Permanently
The requested resource has been assigned a new permanent URI and any future references to this resource should use the new URI.
302 Found
The requested resource resides temporarily under a different URI. Since the redirection might be altered, continue to use the old URI.
303 See Other
The response to the request can be found under a different URI and should be retrieved from the new URI.
304 Not Modified
The document has not been modified since the last request, and the locally cached copy should be used instead.
305 Use Proxy
The requested resource must be accessed through a proxy.
306 (Unused)
Used in a previous version of the protocol.
307 Temporary Redirect
The requested resource resides temporarily under a different URI. Since the redirection may be altered on occasion, continue to use the current URI.
400 Bad Request 
The request could not be understood by the server due to malformed syntax.
401 Unauthorized
The request requires user authentication.
402 Payment Required
This code is reserved for future use.
403 Forbidden
The server understood the request, but is refusing to fulfill it. Typically due to file access permissions on the server.
404 Not Found
The server has not found anything matching the URI.
405 Method Not Allowed
The request method is not allowed. Typically when trying to execute a normal document or display a script.
406 Not Acceptable
According to the client's request, it isn't capable of handling the response.
407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the client must first authenticate itself with the proxy.
408 Request Timeout
The client did not produce a request within the time that the server was prepared to wait.
409 Conflict
The request could not be completed due to a conflict with the current state of the resource. Typically only seen with clients that can manage files on the remote server.
410 Gone
The requested resource is no longer available at the server and no forwarding address is known.
411 Length Required
The server refuses to accept the request without a defined Content-Length header.
412 Precondition Failed
The precondition given in one or more of the header fields failed when it was tested on the server.
413 Request Entity Too Large
The server is refusing to process a request because the request entity is larger than the server is willing or able to process.
414 Request-URI Too Long
The server is refusing to service the request because the requested URI is too long.
415 Unsupported Media Type
The server is refusing to service the request because the client does not support the response format.
416 Requested Range Not Satisfiable
The server could not find the range specified in the Range header.
417 Expectation Failed
The server could not meet the expectation given in the Expect header.
500 Internal Server Error 
The server encountered an unexpected condition which prevented it from fulfilling the request.
501 Not Implemented
The server does not support the functionality required to fulfill the request.
502 Bad Gateway
The server is a gateway or proxy and received an invalid response from the upstream server it accessed in attempting to fulfill the request.
503 Service Unavailable
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server.
504 Gateway Timeout
The server is a gateway or proxy and did not receive a timely response from the upstream server.
505 HTTP Version Not Supported
The server does not support, or refuses to support, the HTTP protocol version that was used in the request.
600 Malformed URI 
The link checking program was not able to parse the URI. This means that the URI was either empty, or not a valid URI (e.g. "Ask at S&E Reference Desk").
601 Connection Timed Out
The server did not respond before the connection timed out.
602 Unknown Error
Some other error occurred in contacting the server, making the request, or parsing the response. This could be due to a mangled or incorrectly formatted response from the server.
603 Could Not Parse Reply
The reply from the server could not be parsed. This could indicate a malformed response, or a non-numerical response code.
604 Protocol Not Supported
This URI uses a protocol that is not currently supported by the linkchecker program. The linkchecker currently only supports http, so ftp, https, gopher, etc. are not supported. Also, some protocols (such as mailto://) cannot be checked.

For more in-depth information, see section 10 of RFC 2616 (which defines what the reponse codes mean).