What do all those error codes (like 404) mean?

It happens to all of us. We're surfing along, and up comes an error. What do all those error codes mean? Here's a quick rundown of the most common:

400 - Bad Request
                   We probably typed in a URL wrong, the server has no clue what we're looking for, or we aren't allowed to have access. Usually, it's a matter of the URL being typing in wrong. Maybe we mixed upper and lowercase letters or something.

401 - Unauthorized Request
                  We tried to get to something on the web server we're not allowed to play with. In other words, we ain't on the party list.

403 - Forbidden
                 We can't access the page. We may not have access (it may require a password), or it may be blocked from our domain.

404 - Not Found                                                                                            The page we were trying to look at was not found on the server. This is probably the most common error we'll come across. What has probably happened is that the web page we were going to has been removed or re-named.

500 - Internal error                                                                                            Usually caused by a CGI error. We 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.