World War II is the largest world conflict ever to happen. The events that unfolded are incredibly numerous and too complex for this one answer. Succinctly put, however, World War II began as a consequence of World War I. WWI ended with the German surrender. The rather harsh sanctions inflicted on Germany after the war by the Treaty of Versailles severely depressed, both economically and otherwise, the German nation. The German people became susceptible to radical political beliefs, which is how Hitler and his Nazis rose to prominence in that country.
Hitler is what history points out as the aggressor of WWII. Through his extreme foreign policies, he worked aggressively to create a more powerful Germany. The scales of power in Europe changed when Hitler ordered the German invasion of Poland in 1939, and war ensued. The overwhelming Allied forces, which eventually included the United States after Japan (Germany's ally) attacked the US at Pearl Harbor, eventually overtook Germany and its allies after several years of intense conflict around the globe.
The war lasted from 1939 to 1945 and the lives of many millions of people were lost. This war is especially noteworthy for being a war where tremendous amounts of civilians perished. The consequences of World War II serve as a reminder of how dangerous political extremism can be.
There were a tremendous number of things that happened in World War II. In this answer, I will give a very bare-bones overview of the war. If you want more detail about some aspect of the war, please ask another question.
World War II began in Europe in September of 1939 when Germany invaded Poland. Germany, which was under Nazi rule, was unhappy with the Treaty of Versailles that ended WWI. Germans felt that the treaty treated them very badly. They wanted to regain their former power and they (particularly the Nazis) wanted to expand and take more territory in Eastern Europe. When Germany invaded Poland, France and the UK entered the war because they did not want Germany to get too strong.
In June of 1941, Hitler committed a major blunder, invading the Soviet Union. Now, Germany and its few allies would have to fight on two fronts. Germany did well at first in its invasion of Russia, but the invasion later turned out to be one of the major factors that caused Germany to lose the war. By the fall of 1941, Germany had defeated and occupied almost all of the countries of Europe that were not on its side.
In December of 1941, Japan attacked the US naval base at Pearl Harbor in Hawaii. Japan wanted an empire in Asia like the empires that the US and some European countries had. It felt that it deserved to be treated as a major power and major powers at that time generally had empires. It wanted an empire so that it could supplement the scant natural resources that existed in Japan. Japan attacked Pearl Harbor (and other places) to prevent the US from interfering as it took an empire in Southeast Asia. The US declared war on Japan and Germany (in another major blunder) declared war on the US.
For a while, Germany and Japan were winning the war. In 1942, however, the tide turned. The Allies (mainly the US, the UK, and the USSR) gradually started to push the Axis back. This took a long time as Germany dominated continental Europe and Japan controlled essentially all of the Pacific outside of Hawaii, New Zealand, and Australia. By June of 1944, the Allies were able to invade Europe. They eventually invaded Germany and Germany surrendered in May of 1945. The Allies (mostly Americans) pushed through Japanese-held islands in the Pacific and eventually reached Japan. They might have had to invade, but Japan surrendered after the US dropped two atomic bombs on Japanese cities, one on August 6 and one on August 9, 1945. When Japan surrendered shortly afterwards, WWII was over.