The Central Powers were Germany, the Ottoman Empire, Bulgaria and the Austrian-Hungarian Empire, while the Triple Entente was composed initially of the Allied Powers Russia, France and Great Britain. The Central Powers crafted the first alliance, and the Allied Powers were designed to counter the Central Powers.
Italy, at first allied with the Central Powers, reneged and made a secret alliance with the Allied Powers. Over time, the Allied Powers swelled to include a total of 25 countries, including the United States after its 1917 entry into the war.
The list of major combatants seems short for a world war. However, several of the involved countries, including Great Britain, France and the Ottoman Empire, were imperialist nations with territories and protectorates that also entered the war. Because of this, it truly was a world conflict.