This is a complex question worthy of a lengthy answer, but this is a short summary.
Germany's invasion of Poland on September 1, 1939 is usually considered the beginning of the war. France and Great Britain declared war against Germany soon thereafter, knowing that Adolf Hitler's intent was to conquer the entire European continent and the British Isles. Italy, Germany's main European ally, had already captured Ethiopia; Hitler's successful invasions of Austria and Czechoslavakia caused many of the other European nations to sign agreements with Germany, nearly all of which Germany later broke. Germany allied itself with Russia for a short time, before invading the huge country in 1941. Japan, who was at war with China and hoped to control most of the Pacific, allied itself with Germany that same year. Hitler's armies captured nearly all of Europe, leaving the British to fight the German Axis alone. When the Japanese attacked Pearl Harbor, Hawaii, in December 1941, the United States entered the war.
The consequences of the war were great: Nearly 75 million people died between 1937-1945. The victorious Allies lost at least 16 million dead, while the Axis nations lost at least eight million. It is estimated that nearly 50 million civilians died during the conflict, including six million Jews systematically exterminated in what became known as the Holocaust. The United States emerged as the richest and most powerful nation in the world, while Russia added new lands following the surrender. Germany and Japan, despite being decisively defeated by the Allied powers, soon rebuilt their economies, becoming among the wealthiest nations on the planet.