World War I had a number of very important impacts on the world. Let us look at a few of them.
First, it helped to make the United States a major power in the world. Before WWI, the US was not a real power either militarily or economically. After the war, it was both. This war was the beginning of a process that would make the 20th century one that was dominated by the US.
Second, it caused Hitler to come to power in Germany. While Hitler did not come to power until 15 years after the war, it was still instrumental in his rise. The war led to the Treaty of Versailles in which Germany was treated very harshly. The anger felt by many Germans about this treaty and the problems that it caused were very important in causing Germans to be willing to follow a man like Hitler. By bringing Hitler to power, it also caused WWII.
Finally, it led to the rise of the Soviet Union. The Russian empire was already teetering before WWI, but its pathetic performance in that war caused it to fall completely. After the Tsar was deposed, the communists ended up coming to power.
By causing these events, WWI helped to shape the rest of the 20th century.