While there were certainly many differences between World War I and World War II, there were also many notable similaities worth examining. This list could be very extensive, but I will mention some of the more striking similarities below.
For starters, the motives of the aggressor nations were largely based on imperialistic nationalism. Trying to conquer new territories and control more resources out of a sense of nationalistic entitlement brought major powers into direct conflict with each other.
In terms of the waging of the war itself, both conflicts are marked by their exceptionally high casualty rates. In both cases, the different militaries took advantage of and developed new technologies that resulted in never before heard of casualties. This includes the use of airpower, submarine warfare, long-range artillery, and improved rifles. World War I saw the use of poison gas, and World War II was the only time nuclear weapons were used in war.
Both wars also resulted in large amounts of civilian casualties. Millions of civilians who were unfortunate enough to be in harm's way died in these conflicts. Sometimes they were even deliberately targeted in an attempt by one side to break the spirit of the other. Additionally, genocide was a factor in each conflict. During WWI, the Ottoman Empire killed as many as 1.5 million Armenians. During WWII, the Germans attempted to eradicate Europe's Jewish, Romani, and homosexual populations.
While the United States was involved in both conflicts, it was late to enter them. Both times, the United States attempted to remain neutral in the fights. While the country did supply aid to Great Britain, the overall mood in the country was in favor of neutrality and isolationism. However, actions by the warring powers would inevitably pull the country into the conflict. When this happened, the United States rapidly mobilized, instituting the draft and transforming its economy to support the war effort.