TOR (The Onion Router) is a network made up of volunteers' PCs, which can be used to encrypt the address of a computer accessing the internet.  TOR PCs can access parts of the internet known as the 'Dark Web' which are used for illegal activity, and cannot be accesed without the correct software.

TOR and similar networks are used to gain an encrypted connection to the internet.  They were initially created so peoples' activities were not tracked on the internet.  For example, people use the networks to talk to other people about personal things that they don't want the rest of the world, and the governments, to potentially see.
Unfortunately such networks are used to do illegal things.  Some of the illegal activities carried out using these networks are sharing illegal images, which would otherwise be seen by the government, and the organisation of protests in countries where protests are not legal.  These activities are carried out on the dark web so that they are not tracked and the people who carry them out are not traced by authorities.