Short answers, pulled from the story.
Gavrilo Princip fatally wounded the heir to the Austro-Hungarian throne and his wife Sophie with two pistol shots in Sarajevo. This single act triggered a month of diplomatic maneuvering known as the July Crisis.
The war began after Austria-Hungary declared war on Serbia on the 28th of July following the assassination on the 28th of June 1914. The conflict ended with the Armistice of the 11th of November 1918 though formal state of war persisted until the Treaty of Versailles signed on the 28th of June 1919.
The Triple Entente included Britain France and Russia while the Triple Alliance consisted of Germany Austria-Hungary and Italy. These alliances formed decades before the war through treaties like the Franco-Russian Alliance in 1894 and the Entente Cordiale in 1904.
One of the deadliest conflicts in history resulted in an estimated 30 million military casualties and 8 million civilian deaths from war-related causes and genocide. Belgium and Serbia were badly damaged while France suffered 1.4 million soldiers dead not counting other casualties.
Four empires disappeared including the Romanovs Hohenzollerns Habsburgs and Ottomans after the war concluded. Numerous nations regained their former independence while new ones were created including Poland Finland the Baltic states Czechoslovakia and Yugoslavia.