HTTP 403:
The 403 Forbidden HTTP status code indicates that the client was able to communicate with the server, but the server doesn't let the user access what was requested.
An example of descriptive English text that can often accompany such a response is You don't have permission to access /forbidden on this server.
This indicates a fundamental access problem, which may be difficult to resolve, because the HTTP protocol allows the Web server to give this response without providing any reason at all. So the 403 error is equivalent to a blanket 'NO' by your Web server - with no further discussion allowed, and according to the RFC specifications, with a 403 error code, the request should not be repeated.
This response may indicate a resource that requires authentication, such as basic access authentication or digest access authentication through the Authorization request header, or some other form of authentication which is managed through user scripts in a programming language such as PHP. However, the HTTP specification states that 403 should only be used when authorization will not help. The 401 Unauthorized code is a better choice for this situation. Google App Engine has also re-purposed this error code as "403 Over Quota", used when an application has exceeded one or more Google-specified resource quotas.
This response is returned by the Apache web server when directory listings have been turned off. Microsoft IIS responds in the same way when Active Directory listings are denied.
It seems to now be commonly used by certain ISP's to block access to certain sites which the ISP considers "bad". This is a certain form of censorship. The ISP may decide the site is pornographic for instance, or, is a front for an "illegal" business located outside of the USA and selling drugs illegally. This may happen to avoid liability or to comply with Federal laws.
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