There were a number of countries involved in World War One. This war was largely a clash of empires, so many of these were colonies of the British, French and German empires that provided additional military support. As such, these were the countries that provided the most notable military contribution to their respective alliances of the Triple Entente and the Triple Alliance.
Russia had the largest army of the war, so in that respect may have been a key ally of the Entente. France's alliance with Russia ensured that Germany had two fronts with a good number of their soldiers involved in the East. However, the Russian army was not especially well equipped; and they had a number of military defeats such as the Battle of Tannenberg. After the fall of the tsar and Russian Revolution, the Bolsheviks entered peace talks with Germany; and they signed the Treaty of Brest-Litovsk before the war ended.
France and Empire
The French held the line in Western Europe, with the support of Britain, for most of the war. There Germany could not break through as they were twice defeated by the French at the Battle of the Marne. As such, the French eventually emerged victorious in 1918 as they pushed the Germans back. In addition, the French Empire included many African countries such as Ivory Coast, Guinea, and Senegal.
Britain and Empire
The British Empire joined the war in 1914 after Germany invaded Belgium. The British provided military support on the Western front, halting the German advances, and in 1918 their tanks were very effective in pushing the Germans back at the Battle of Amiens. At sea the Royal Navy provided an effective German economic blockade for much of the war, which resulted in shortages in Germany. Beyond Europe, British Empire troops were also involved in the Gallipoli Campaign that backfired. A few of their colonies provided military resources for Britain such as Australia, New Zealand, India, Kuwait and Canada.
The United States of America remained isolationist for most of the war. However, they joined the war in 1917 after Germany U-boats sank U.S. shipping. As such, by 1918 their troops were arriving in Europe in numbers; and re-enforced the Entente during the final advances in the West.
The Italians were never the most reliable of Entente allies and actually began WW1 as a part of the Triple Alliance. However, they later joined the Entente in 1915 and established a front with the Austro-Hungarian army. By 1918 their own advances played some part in the defeat of the Austro-Hungarian Empire.
Japan provided naval support for the Entente. In addition, they also advanced on German land-based targets in Asia.
World War One began after the Serbian nationalist assassinated the Archduke Ferdinand. With this, the Austrians declared war on them. However, Serbia also had Russian support in the Balkans.
The Triple Alliance
Germany provided the most significant military resources of the Triple Alliance. The Germans were involved on two fronts in Eastern and Western Europe, and by 1917 had largely defeated the Russians in the East. However, they could not break the Anglo-Franco front in Western Europe; and when the USA joined the war in 1918 the German army retreated. At sea the Imperial German Navy was also one of the most formidable; however, they were largely defeated by the larger Royal Navy and remained in port for most of the war except at the Battle of Jutland. In addition to this, Germany had a more modest African empire, which included Togoland, that was lost to the Entente during the conflict.
Austria and Hungary dominated the empire of Austria-Hungary. They were a loyal and key German ally, and it was the assassination of Archduke Ferdinand that provided the spark for World War One. Their primary fronts were with the Italians and Serbs, but they also provided some additional support for Germany's front with Russia. By 1918, Austria-Hungary was all but defeated by the Entente.
Ottoman Turkey was the empire of modern-day Turkey. They were primarily involved in the Middle East. There the Turks were up against British Empire troops such as during their great victory at Gallipoli that defeated the British advance. However, the Arab revolt, inspired by the likes of Lawrence, turned the tide against the Turks from 1916 onwards; and the empire collapsed after the war.
So these were the main countries and empires involved in World War One. As the Germans, British, French, Turks and Austrians all dragged their empires into the war, it involved many more countries that were former colonies.