That's a very broad question depending on what you include in the definition of impact, but let me get you started on what I believe to be a few important impacts.
Human Cost - 16 million people, both military and civilian, died in this war. Entire generations of young men almost wiped away. In 1919, the year after the war was over in France, there were 15 women for every man between the ages of 18 and 30. All of the lost potential, all of the writers, artists, teachers, inventors and leaders that were killed were permanently denied to the future of the nations.
Imperial Realignment - The Austro-Hungarian Empire was broken up and drastically shrunk, while Poland, Czechoslovakia and Yugoslavia were all born as new nations. The nation of Germany was also smaller, and the Ottoman's of Turkey were finished. The Kaiser went into exile, and Germany plunged into economic and political chaos.
Pacifism/Despair - The postwar generation in Europe really believed it had been the "War to End All Wars" in that they never wanted to experience such a catastrophe again, and for a short time during the 1920s, a wave of pacifist literature and art fluorished in France and Germany. Others, damaged by the war physically or emotionally, or having lost loved ones, had lost faith in humanity given the terrible destruction the war brought.