There are multiple HTTP codes that you may come across. Below you can find a summary of the most popular HTTP error codes:
"200 OK" - You will not see this code when browsing a regular page. It means that the requested document has been processed and sent to you.
"301 Moved Permanently" - Often web designers create the so-called 301 redirects. This means that instead of the file you are browsing to, the server will point you to a different directory or a file set in the redirect rules.
"400 Bad Request" - The Web server considers the data stream sent by the client (your Web browser) 'malformed' i.e. it has not sent a complete HTTP request. In such cases the web server is unable to process the request. Almost always this error is caused by bad programming.
"401 Unauthorized" - You are most probably trying to access a password protected directory using wrong credentials. In such cases you should check whether you are using the correct login details. If you wish to have your credentials reset, you can do this using the "Password Protect Directories" tool in your cPanel.
"403 Forbidden" - This error message means that you are trying to open a folder or a file you do not have access to. If you are receiving this error message on your website, you should check the .htaccess files in your web hosting account for any restrictive rules.
"404 Not Found" - You are browsing to a non-existent page or a folder. In such cases you should check if the link you are trying to access is correct. In addition, if any SEF functionality on your pages is not working correctly, your application may forward you to the wrong URL.
"500 Internal Server Error" - The most common reasons for this error are bad script code, an invalid .htaccess file, wrong file/folder permissions. For more information on that matter, you can check on this article.