A group which calls itself Islamic State (IS) has claimed responsibility for a number of recent attacks which have shocked the world.
But what do we know about them?
IS is a group based in the Middle East, mainly in Iraq and Syria, whose members have extreme religious beliefs. They use brutal violence against anyone who doesn't agree with their views.
They claim to follow the religion of Islam, but many people say their beliefs of hatred and violence have little to do with what the majority of Muslims believe.
From early 2014, IS grew quickly, using the internet and social media to recruit followers and spread fear.
It was able to take over large parts of Syria and neighbouring Iraq, declaring this their own state, and punish anyone who doesn't follow their strict rules.
What does unite most of the world is the desire to stop IS because of how violent their methods are.
In 2015, the UK joined the US and France in bombing some areas of Syria which are controlled by IS.
The UK's former Prime Minister David Cameron argued in Parliament that bombing IS was one way to try to stop them.
But some people feel that the bombs affect too many innocent people living in Syria and that the UK shouldn't be involved.