The Internet Watch Foundation works with British internet service providers to remove child pornography and other potentially illegal content from the web.
An independent, self-regulatory body, which is funded by the EU, it sets itself the goal of making the internet safer.
It assesses potentially illegal sites according to UK law, which states that it is offence to take, distribute or show indecent photographs of children under the age of 18.
Those deemed to be illegal are added to a list, which is sent to companies that have committed themselves to removing unacceptable material from the web.
At any one time it says that between 800 and 1,200 sites feature on the list, which is updated twice a day.
The IWF does not notify the sites' owners that their websites are on the list, but it does operate an appeal process if anyone wishes to challenge the decision.
Firms working with the IWF are not obliged to remove the content but generally do so by using filtering or blocking technology.
Its members include AOL, Talk Talk, Virgin Media, Sky and PlusNet.
The body operates an online "hotline" to enable anyone worried about a potentially illegal site to report their concerns.
Internet users can register their complaints at www.iwf.org.uk
The reports received by the IWF's hotline are assessed by Internet Content Analysts who have comprehensive and in-depth training on relevant UK legislation.