HTTP Status Codes give us clues about the result of our operation. They can be categorized under 5 titles:

1xx Informational 

2xx Successful 

3xx Redirection 

4xx Client Error 

5xx Server Error 

Here are some of the most common ones that hundreds of millions of people encounter every day:

200 OK

In the case of the HTTP status 200 OK, the web server processed the request successfully and transmitted content back to you. Every time you visit a web page this status is encountered by your web browser but it’s just not shown to you.

404 Not Found

This is a problematic one. 404 is an error and it means requested page, file, media or whatever it is can not be found. It might have been moved or maybe address you have is wrong?

500 Internal Server Error

500 means your request was valid however, it can not be processed. It can be a server problem or a restriction, it could be a tiny glitch, a wrong setting, maybe an intentional problem caused by the administrator or even a memory problem from the server.

400 Bad Request

This means your request is not understood by the server. Usually it means your syntax is problematic unless the server has a serious technical problem which is also a possibility.

502 Bad Gateway

Usually occurs due to configuration errors of settings. It can be a caused by your modem, firewall or router.

 You can learn more about HTTP Error and Status codes in this Wikipedia page.