The First World War was a world war because it involved several countries who had empires. So when the mother country of the empire became involved in war,

In World War I the war started because each of the countries of Europe had treaties with others that said if they were attacked, the other treaty countries would come to their aid. This was a disaster. It meant that as soon as there was a small argument between small countries all the bigger countries were automatically drawn in.

The war started when a member of the Royal Family of Bosnia was shot by an assassin. He was shot with the intention of triggering all the treaties and starting a war. The people who did it hoped to cause chaos and gain control themselves.

The First World War brought down the Russian empire and gave rise to Communism.

The First World War made Germany weak and allowed Adolf Hitler to come to power on the promise of getting the pride of Germany back. This directly led to the Second World War.

The First World War was fought by people who had a heritage of fighting with armies face to face, but they had developed weapons that changed everything. But no one knew how to change the way to fight a war. So they all ended up digging long trenches across northern France and Belgium where they fired the new weapons at one another and killed millions for no purpose.