World War I, also referred to as the Great War, and the First World War, was an armed military conflict that lasted from July, 1914 to November, 1918. It involved the Allied Powers of Europe (England & France), as well as Russia, against the Central Powers of Germany & Austria-Hungary, and the Ottoman Empire. The war resulted in an Allied victory at the Treaty of Versailles in France, which was signed on June 28th, 1919. The war led to the formation of the League of Nations.
Obviously, nobody who actually served during the war called it World War I. It was only known as the Great War. It wasn't until World War II, that people decided to attach a numeric qualifier to it. I mean... who would'a thought all this craziness would happen twice?