World War I differed from previous wars in a number of ways. Let us examine some of the most important of these ways.
First, it was simply a bigger war. This was the first war to include so many of the major powers of the world. The Napoleonic Wars had come close, but they did not include Asian countries like Japan and the Ottoman Empire (if we can call that an Asian country) or former British colonies like the United States and Australia (which was at this time a dominion and not a fully independent country).
Second, it was a war that had all sorts of modern technologies not used before in wars. These included such things as submarines and airplanes, machine guns and poison gas. It also, by the end, included tanks.
Third, it can be seen as the first total, industrial war. Because of the new technology and the scale of the war, this was a war whose combatants needed to mobilize their entire populations in one way or another.
WWI, then, was the first war to be so big and so technological that it can be called a total war that involved most of the major powers in the world.