3. Internet Filters

Internet filters, technically known as content-control software, are used in order to filter out what internet information can be made visible to a user.  These are used to prevent information from being delivered to the computer so as to prevent any unwanted virus downloads.  Content-control software is commonly used in schools, to prevent students from accessing certain information, businesses, and libraries.  Certain countries also use content-control software in order to censor certain information from its people.

 Client-side filter software is installed on the computer and used to moderately filter out certain information, but can be overridden by someone who has a password.  Content-limited ISPs are internet service providers that automatically filter certain information, so that anyone who subscribes to that service provider can not access that information.  Server-filter sides are used for entire institutions, such as schools or libraries, and the settings are customized to fit all computers in the network by the institution. 

http://en.wikipedia.org/wiki/Content-control_software