What is filtering?

Internet content filtering systems prevent or block users’ access to unsuitable material online. When the filtering system is turned on, users cannot open or link to sites that the filtering system recognises as unsuitable. Various methods for filtering content are available, such as keyword matching or blocking, site blocking, keyboard blocking, or protocol-blocking systems. Many filtering systems will also provide facilities to filter or block applications also, such as email and file-sharing applications.
The main aim of filtering content is minimise the risk of users accessing inappropriate or illegal content online. Although some categories of content will clearly be inappropriate for all children and young people – for example those promoting pornography, violence, racist or criminal content.

Reference:
Found at http://schools.becta.org.uk/index.php?section=is&catcode=ss_to_es_pys_fc_03