Common HTTP states (304,)

One, 1XX (temporary response)

A status code that represents a temporary response and requires the requestor to continue the operation.

100 (Continue)	The requestor shall continue to make the request. The server returns this code to indicate that the first part of the request has been received and is waiting for the remainder.
101 (Switching protocol)	The requestor has asked the server to switch protocols and the server has confirmed and is ready to switch.

Second, 2XX (success)

Indicates the status code of the request was successfully processed.

200 (success)	The server has successfully processed the request. Typically, this indicates that the server provided the requested Web page. If this status code is displayed for your robots.txt file, it means that Googlebot has successfully retrieved the file.
201 (created)	The request was successful and the server created a new resource.
202 (accepted)	The server has accepted the request but has not yet processed it.
203 (non-authorised information)	The server has successfully processed the request, but the information returned may be from another source.
204 (no content)	The server successfully processed the request, but did not return any content.
205 (Reset content)	The server successfully processed the request, but did not return any content. Unlike the 204 response, this response requires the requestor to reset the document view (for example, to clear the form contents to enter new content).
206 (partial content)	The server successfully processed a partial GET request.

Three, 3XX (redirect)

Further action is required to complete the request. Typically, these status codes are used for redirection. Google recommends that you use redirects no more than 5 times per request. You can use the Webmaster tools to see if Googlebot is having trouble crawling the redirected pages. The network crawl page under diagnosis lists URLs that Googlebot cannot crawl due to redirection errors.

300 (multiple options)	For requests, the server can perform a variety of operations. The server can select an action based on the requestor (user agent) or provide a list of actions for the requestor to select.
301 (Permanent movement)	The requested Web page is permanently moved to a new location. When the server returns this response (a response to a GET or head request), the requestor is automatically forwarded to the new location. You should use this code to tell Googlebot that a Web page or Web site has been permanently moved to a new location.
302 (Temporary Move)	The server currently responds to a request from a Web page in a different location, but the requestor should continue to use the original location to respond to subsequent requests. This code, similar to the 301 code that responds to the get and head requests, automatically transfers the requestor to a different location, but you should not use this code to tell Googlebot that a Web page or site has moved because Googlebot continues to crawl the original location and index it.
303 (See other locations)	The server returns this code when the requestor should use a separate GET request for a different location to retrieve the response. For all requests except head, the server automatically goes to a different location.
304 (not modified)	The requested page has not been modified since the last request. When the server returns this response, the Web page content is not returned. If the page has not changed since the requestor last requested it, you should configure the server to return this response (become the If-modified-since HTTP header). The server can tell Googlebot that the webpage has not changed since the last crawl, thus saving bandwidth and overhead.
305 (use proxy)	The requestor can only use the proxy to access the requested Web page. If the server returns this response, it also indicates that the requestor should use the proxy.
307 (Temporary redirect)	The server currently responds to a request from a Web page in a different location, but the requestor continues to use the original location to respond to subsequent requests. This code, like the <a href = answer.py?answer=>301</a> code that responds to the get and head requests, automatically transfers the requestor to a different location. However, you should not use this code to tell Googlebot that a page or site has moved because Googlebot will continue to crawl the original location and compile it.

Four, 4XX (request error)

These status codes indicate a possible error in the request and hinder the processing of the server.

400 (Bad Request)	The server does not understand the syntax of the request.
401 (not authorized)	Request authentication required. The server may return this response for Web pages that are requested after logging in.
403 (Forbidden)	The server rejected the request. If you see this status code when Googlebot tries to crawl a valid webpage on your site (you can see this information on the Web crawl page under Google Webmaster Tools diagnostics), your server or host may have denied Googlebot access.
404 (Not Found)	The server could not find the requested Web page. For example, this code is often returned for Web pages that do not exist on the server. If you do not have a robots.txt file on your site and you see this status code on the robots.txt page of the Googlebot Webmaster Tools Diagnostic tab, this is the correct status code. However, if you have a robots.txt file and you see this status code, your robots.txt file may be named incorrectly or in the wrong location (the file should be in the top-level domain named robots.txt). If you see this status code for Googlebot crawled URLs (on the HTTP error page of the Diagnostics tab), it means that Googlebot follows a potentially invalid link to another page (a link that is an old link or incorrectly entered).
405 (method Disabled)	Disables the method specified in the request.
406 (not accepted)	The requested content attribute could not be used to respond to the requested Web page.
407 (requires proxy authorization)	This status code is similar to <a href=answer.py?answer=35128>401 (unauthorized) </a>, but specifies that the requestor should authorize the use of the proxy. If the server returns this response, it also indicates that the requestor should use the proxy.
408 (Request timed out)	A timeout occurred while the server was waiting for a request.
409 (conflict)	The server encountered a conflict while completing the request. The server must include information about the conflict in the response. This code may be returned by the server in response to a PUT request that conflicts with the previous request, as well as a list of differences of two requests.
410 (Deleted)	If the requested resource has been permanently deleted, the server returns this response. This code is similar to the 404 (not Found) code, but is sometimes used to replace the 404 code in cases where the resource existed before and now does not exist. If the resource has been permanently moved, you should use 301 to specify a new location for the resource.
411 (valid length required)	The server does not accept requests that do not contain a valid Content-Length header field.
412 (precondition not met)	The server did not meet one of the prerequisites set by the requestor in the request.
413 (Request entity too Large)	The server cannot process the request because the request entity is too large to exceed the processing power of the server.
414 (The requested URI is too long)	The requested URI (usually the URL) is too long and the server cannot process it.
415 (Unsupported media type)	The requested format is not supported by the requested page.
416 (Request scope does not meet the requirements)	If the page cannot provide the requested range, the server returns this status code.
417 (expectations not met)	The server does not meet the requirements for the expected Request header field.

Five, 5XX (server error)

These status codes indicate that an internal error occurred while the server was processing the request. These errors may be errors on the server itself, not the request error.

500 (server internal error)	The server encountered an error and could not complete the request.
501 (not yet implemented)	The server does not have the capability to complete the request. For example, this code may be returned when the server does not recognize the request method.
502 (Wrong Gateway)	The server received an invalid response from the upstream server as a gateway or proxy.
503 (service not available)	The server is not currently available (due to overloading or downtime maintenance). Typically, this is just a paused state.
504 (Gateway Timeout)	The server acts as a gateway or proxy, but does not receive requests from the upstream server in a timely manner.
505 (HTTP version not supported)	The server does not support the HTTP protocol version in the request.

Common HTTP states (304,)