The assassination of Archduke Franz Ferdinand of Austria-Hungary was the spark that is credited with setting off World War I.
The Archduke was the heir to the throne of Austria-Hungary. This was a multiethnic empire whose rulers were ethnically German. Many of the people that they ruled were Slavs. This was the case in the Balkans, in places that later became Yugoslavia and have now broken up into countries such as Serbia, Croatia, and Bosnia. The Slavs did not like being ruled by people who were ethnically different from them. Therefore, there was a Slavic nationalist movement in that part of the empire. It was supported by Serbia, then an independent country, and Russia, both of which were Slavic countries.
On June 28, 1914, the Archduke was visiting Sarajevo, in what is now Bosnia. There, he was assassinated by a Slavic nationalist. In response, Austria-Hungary, which was looking for a reason to go to war with Serbia anyway, issued an unreasonable ultimatum to that country. When Serbia would not accept all the terms of the ultimatum, Austria-Hungary went to war. This triggered the more general war that we now know as WWI.