"Whitelisting" software blocks out all Internet content, except expressly authorised sites on a "whitelist". This technique is highly limitative, and runs contrary to the logic of the Internet. It is however very safe, and has been used, in particular in the school environment.
Filtering software follows three main models: "blacklisting" (where access to listed sites is blocked), "whitelisting" (where access is only possible to listed sites) and "neutral labelling" (where sites are labelled or rated, but it is up to the user to decide how to use the label or rating).