400 - Bad Request - You probably typed in a URL wrong, the server has no clue what you're looking for, or you aren't allowed to have access. Usually, it's a matter of the URL being typing in wrong. Maybe you mixed upper and lowercase letters or something.
401- Unauthorized Request - you tried to get to something on the web server you're not allowed to play with. In other words, you ain't on the party list.
403 - Forbidden - You can't access the page. You may not have access (it may require a password), or it may be blocked from your domain.
404- Not Found - The page you were trying to look at was not found on the server. This is probably the most common error you'll come across. What has probably happened is that the web page you were going to has been removed or re-named.
Keep going backwards in the URL and you'll usually come up with the site you're after. Then you can look for the lost page from there.
Another possibility is that your Internet connection has gone dead. Try re-logging on.
500 - Internal error - Usually caused by a CGI error. You fill out a form, but the script used to process it is not working properly.
503 - Service Unavailable - The server may be overloaded, down, or have other similar problems. Try later.