How was World War II a "total war"?

According to the Oxford dictionary, a total war is defined as follows:

A war that is unrestricted in terms of the weapons used, the territory or combatants involved, or the objectives pursued, especially one in which the laws of war are disregarded.

With this definition in mind, we can easily see how World War II was a total war. It was a winner-take-all contest. Whichever side lost would be forced into a total, unconditional surrender. It was not a limited war with limited objectives that used limited firepower to achieve a goal. Instead, the goal was the total crushing of the enemy.

Neither side held back on the weaponry they used, and in fact, both sides battled to develop ever more lethal weapons. The Nazis developed V-2 rockets that could be launched from the ground and carry explosives long distances. The U.S. developed the atomic bomb, which delivered a stunning and unprecedented level of destruction.

The war also did not make distinctions between combatants and non-combatants. The Nazis killed many civilians in Poland and the Soviet Union to try to clear the space for Germans, and did not care how many civilians their blitzkrieg against England took out. Likewise, the Allies used air warfare, including firebombs, against cities largely built of wood. They pulverized these cities with no concern for civilian deaths, wanting to break the will of the enemy. Another disregard of non-combatant zones took place when the U.S. atom bombed Hiroshima and Nagasaki in order to avoid ground combat in Japan (which may have taken as many civilian lives).

The fire bombings of civilians were considered a disregard of the rules of war, as were the Nazi's mass execution of Soviet prisoners of war near the war's end. Finally, the war wasn't over until the Allies achieved the total defeat of Germany and Japan.

World War II was a total war in the sense that the countries involved employed all their resources in order to help the war effort. Just as this war spread over a greater geographical extent than other wars, its nature, too, became all-encompassing. Unlike previous wars, the scale of involvement spread far beyond just the use of armies and navies. It was not just the actual fighting that mattered, but the continuous manufacturing of arms and equipment to help the war effort. This was a highly mechanized war, more so than previous conflicts, and consequently the demand for modern, highly specialized equipment - fighter planes, guns, bombs, and so on - was huge. Additionally, in countries like Britain, food and clothes were rationed, and many people helped out in volunteer organizations all over the country to boost the war effort. The war was viewed very much, then, as a national affair. This feeling was increased by the use of modern communications, principally radio broadcasts which kept the people updated on what was happening and, just as importantly, endeavoured to boost the national morale by constant appeals to patriotism and collective strength.

Of course, there was a whole other and much grimmer side to this unprecedented scale of civilian involvement in the form of frequent air-raids on major cities. Civilians became a widespread target in this war. There was the London Blitz, the destruction of Dresden, and, finally, at the end stage of the conflict, the dropping of the first nuclear bombs on the Japanese cities of Hiroshima and Nagasaki.

Explain how WWII was both a "world" and "total" war.

World War II was a global war in the sense that it included major participants from all over the world. Japan allied with Germany and Italy because Japan wanted British and French colonies in Asia. The major battlefields of World War II were both in Asia and Europe; no war before this one had major battlefields this far apart. Sea lanes also figured prominently into this war as the merchant marine of all the major participants in the war were targeted.

World War II was also a total war in that civilians were targeted as well as combatants. Civilians produced goods for and in factories, so they were deemed fair game in this war. Entire civilian centers such as Dresden, London, and Tokyo were bombed in order to either force governments to capitulate or to hamper the enemy's war production. The United States bombed Japanese cities indiscriminately because a great deal of Japanese war production took place in people's homes. Germany viewed Soviet and Polish civilians as war targets because these people would either have to be enslaved or destroyed for Germany to get more living space. Imperial Japan took the same attitude towards Chinese civilians.

The total and global aspects of World War II combined to make it the most destructive war in history.

World War II was a total, world war because it involved countries around the entire world and involved them in such a way that they had to use their entire economies and populations to participate in the war.

Unlike World War I, WWII was a truly world war.  WWI involved a few non-European peoples, but largely because those peoples were connected to European imperial powers.  Japan was officially in the war but did very little.  The vast majority of the fighting occurred on the European continent.  WWII was not like this.  Japan was, of course, completely involved in this war.  Fighting occurred in China and on many Pacific islands.  It took place in Northern Africa as well.  It was a much more widespread war than WWI.

WWII also involved more of the belligerents’ populations.  In the US, for example, there was rationing because many commodities were needed for the war effort.  Women went to work in roles that had previously been closed to them.  The same sorts of things happened in Japan, for instance.  In Japan and Germany and England, civilians far from battlefields were killed in large numbers by bombing raids.  These sorts of things were much less common in WWI.

For these reasons, WWII was global and total in a way WWI had not been.