Short answers, pulled from the story.
The Central Powers consisted of four main members: the German Empire, Austria-Hungary, the Ottoman Empire, and the Kingdom of Bulgaria. Together they were also called the Quadruple Alliance. The group was sometimes referred to as the Central Empires.
The name refers to the geographic location of its members. All four nations were situated between the Russian Empire to the east and France and the United Kingdom to the west, placing them at the center of the European map. Each member used a version of the name in its own language, except Turkish-speakers, who called the alliance the 'Allied States.'
Austria-Hungary joined on the 28th of July 1914, Germany on the 1st of August 1914, the Ottoman Empire on the 29th of October 1914, and Bulgaria on the 14th of October 1915. Bulgaria was the last to enter the war, declaring war on Serbia to reclaim the Vardar Macedonia region.
The Central Powers mobilized a combined total of over 25 million soldiers across all four members. Nearly 4.4 million were killed in action, and total casualties including wounded and missing exceeded 18.8 million. Austria-Hungary had the highest casualty rate at 94 percent of forces mobilized.
Bulgaria aligned with the Central Powers in October 1915 because they guaranteed its right to reclaim the Vardar Macedonia region, which Serbia had held since the Balkan Wars of 1912-1913. The Allies offered only distant and hypothetical promises by comparison. Bulgaria formally signed an alliance treaty with Germany, Austria-Hungary, and the Ottoman Empire before declaring war on Serbia.
The Central Powers were dealt with in separate treaties rather than a single unified agreement. Germany was required to pay 132 billion gold marks, reduce its army to 100,000 men and its navy to 15,000 sailors, and was forbidden from having tanks, submarines, or an air force. Hungary lost 72 percent of its total land area under the Treaty of Trianon.