The role of the United States in the First World War was fairly limited, with some historians giving our military role more of a psychological or motivational role than actually a functional one. Knowing help was on the way was a huge boost to the Allied morale. Of course, American troops fought in a number of battles and obviously served a purpose in the victory, but their effect on the Allied victory wasn't nearly as important as the help the US provided economically.
Economically speaking, the United States played an integral role in World War I. Some estimates contend that the United States gave the allies nearly ten billion dollars in credits to be used primarily on raw materials to drive the European war machines. In addition, the US withheld raw materials to combative nations often times with crippling effects. The United States manufactured a great number of weapons for use in the war, with some estimates as high as 60% of the ships needed to face the U-boat threat of Germany. The US presence on in the Atlantic also kept other nations trading with the Allies, that otherwise wouldn't have had the might to contend with that obstacle. The United States provided the financing, materials, and weapons that allowed the Allies to finish the war in victory.