There were many differences between the two wars. One difference between the two wars was how they started. WWI started when the Austrian archduke was killed in Sarajevo, thus causing the alliance groups to come to each other's aid. WWII was caused by German imperialism and its invasion of Poland.
Another difference between the two conflicts was the technology used by the combatants. WWI on the Western front was fought in a series of bloody trench battles between 1914 and mid-1918. Fortifications and automatic weapons made advances costly. Neither side fully utilized the rudimentary tanks of the period. Both sides used poison gas with limited results. Both sides also used aircraft but these were seldom used in support of ground operations. In WWII Germany perfected mechanized warfare to the point that it was able to overrun Poland, the Low Countries, and France relatively quickly. Aircraft supported ground operations. Tanks were faster and also supported infantry. The main challenge of WWII for the Germans was to keep the armies supplied with food and petroleum. This was one of the key failures for the Germans and a reason why they lost WWII.
Still yet another difference was the casualty totals for the wars. While both sides suffered immensely in WWI, the casualty totals are much higher for WWII. This is due to the global scope of the war—Japan's active work for the Axis powers easily added millions of Asians to the casualty lists who would not have been likely thirty years previously. Another reason casualties were so high was the total nature of WWII. Entire cities were leveled as civilians were viewed as combatants due to their work in war industries and food production. Dresden, Tokyo, and London all suffered major losses during the war. Perhaps the most iconic loss of the war came from the bombings of Hiroshima and Nagasaki at the end of WWII.
While there are others, the final difference I would like to point out would be how the wars ended. WWI ended with a German armistice. Germany was actually invaded and Berlin taken at the end of WWII. Japan and Italy also had to sign treaties of unconditional surrender in this war. The end of WWI saw the end of the Austro-Hungarian and Ottoman Empires. These two empires had existed for hundreds of years. The Russian Empire also collapsed during the war. WWII did not see the collapse of any empires, though the Japanese governmental system was restructured in order to make it more democratic. The end of WWI saw the birth of the League of Nations, but it failed due to lack of enforcement and lack of American involvement. The end of WWII saw the birth of the United Nations. It continues to be an organization for peace today.