Just to clarify, the first answer correctly identifies World War II as the conflict that was the bloodiest. Remember that it occurred across three continents, involved dozens of countries including most of Europe and Asia, and the war on the Eastern Front between the Soviet Union and Germany was particularly brutal and bloody. Most histories of the war do not do justice to that part of World War II in my opinion.
In addition to the German concentration camps, the Japanese were horribly brutal in Korea, China, Indochina and the Philippines, and hundreds of thousands of civilians died under their occupation. There were 300,000 estimated killed in the Rape of Nanking alone. The first post accurately also points out the enormous human cost of strategic and atomic bombing by the Allies.
The post above mentions the Korean War, which, while very bloody and tragic, comes nowhere close to the scale or loss of World War II.
By far, World War II killed more people than any other war in human history. The people killed in this war included soldiers, of course. But this was also the war in which more civilians were killed than any other war.
Huge numbers of civilians were killed by the strategic bombing raids used by the Germans against England and, especially, by the Allies against Germany and Japan.
In addition, millions of civilians were killed in the Holocaust.
It is unclear exactly how many millions of people died in the war, but most estimates range from 50 to 60 million people. The hitler.org link (no, it's not a pro-Hitler site) shows the number of dead by country.
World war 2
As for more the Korean war KILLED a trillion of people. In all about 4 trillion people died. The UN(United Nations)was on the Korean side and China and other countries were on the North Korea side. People captured enemies and the war was aggresive. I feel sorry for them. This war was scary and shocking I say. IF I was in this war I'll scream until I died of those smokes from the taks and gunshots.WOW!
only 54,246 died compared to the 50 million to 80 million in ww2