To begin discussing what HTTP Status Codes are, it is perhaps important to quickly delve into what HTTP means. HTTP stands for Hypertext Transfer Protocol, and this is a primary application protocol on the world wide web, used for distribution of collaborative hypermedia. For example, hyperlinks like this are possible because of HTTP.
So HTTP Status Codes are a set of codes that go between the server and the device it is communicating with that explain or clarify what is occurring within the hypermedia. The requesting device requires the clarity of this information from the host server so that the requested task can either be completed, or the reason for the task's failure can be communicated to the requesting device, and by extension the end user.
There are myriad HTTP Status Codes, some more common than others. A complete list can be found on the IANA website. The IANA are responsible for the standardization and regulation of these codes.
Here are a few of the most common HTTP Status Codes you may encounter:
200 OK - This means the connection has been successful.
204 No Content - This code indicates a successful connection, but also indicates that there is no additional metadata or content to be sent to the requesting device.
301 Moved Permanently - This code indicates that the requested content has been moved to a new location, and assuming the hyperlinking is correct, the server should redirect to the content's new location.
400 Bad Request - This indicates that the server did not understand the request and was unable to process the protocol.
401 Unauthorized - This common error message shows that the requesting device does not have the correct permissions to view the requested content on the server.
403 Forbidden - The server refuses the requesting device's query for unspecified reasons.
404 Not Found - The server was unable to find the requested content.