The spark that started World War I was the assassination of Austria's Archduke Franz Ferdinand and his wife Sophie. The assassination occurred on June 28, 1914 while Ferdinand was visiting the city of Sarajevo in the Austro-Hungarian province of Bosnia-Herzegovina.Although Archduke Franz Ferdinand, the nephew of Austria's emperor and heir-apparent to the throne, was not very well liked by most, his assassination by a Serb nationalist was viewed as a great excuse to attack Austria-Hungary's troublesome neighbor, Serbia.
However, instead of reacting quickly to the incident, Austria-Hungary made sure they had the backing of Germany, with whom they had a treaty, before they proceeded. This gave Serbia time to get the backing of Russia, with whom they had a treaty.
The calls for back-up didn't end there. Russia also had a treaty with France and Britain.
This meant that by the time Austria-Hungary officially declared war on Serbia on July 28, 1914, an entire month after the assassination, much of Europe had already become entangled in the dispute.
At the start of the war, these were the major players (more countries joined the war later):
Entente (a.k.a. Triple Entente, a.k.a the Allies, ): France, the United Kingdom, Russia
Central Powers: Germany and Austria-Hungary, Ottoman Empire (Turkey)