Terrorism is the use of violence,fear and intimidation to publicise a political cause.
In the 1970s and 80s, the best known terrorist groups was the IRA (Irish Republican Army), which used violence to campaign for Irish independence from the UK.
More recently, Al-Qaeda and Isis have been responsible for acts of terror in Europe.
Modern day terrorists use the internet to communicate and spread their message.
Other groups also use terrorism to further their cause, for example animal rightsactivists.