The term HTTP status code is actually the common term for the HTTP status line that includes both the HTTP status code and the HTTP reason phrase.
For example, the HTTP status line 500: Internal Server Error is made up of the HTTP status code of 500 and the HTTP reason phrase of Internal Server Error.
Two major groups of HTTP status code errors exist:
4xx Client Error
This group of HTTP status codes include those where the request for a web page or other resource contains bad syntax or cannot be filled for some other reason, presumable by fault of the client (the web surfer).
5xx Server Error
This group of HTTP status codes include those where the request for a web page or other resource is understood by the website's server but is incapable of filling it for some reason.
See a complete list of errors on my HTTP Status Code Errors page.
Other HTTP status codes exist in addition to 4xx and 5xx codes. There are also 1xx, 2xx, and 3xx codes that are informational, confirm success, or dictate a redirection, respectively. These additional types of HTTP status codes aren't errors so you shouldn't be alerted about them in the browser.
Important: An HTTP status code is not the same as a Device Manager error code or a system error code. Some system error codes share code numbers with HTTP status codes but they are different errors with completely different associated error messages and meanings.