Short answers, pulled from the story.
The Warlord Era lasted from 1916 to 1928. It began after the death of Yuan Shikai on the 6th of June 1916, which created a power vacuum quickly filled by rival military commanders. It officially ended on the 29th of December 1928, when Zhang Xueliang accepted the authority of Chiang Kai-shek's Nationalist government.
The most powerful northern warlords were Feng Guozhang of the Zhili clique, Duan Qirui of the Anhui clique, and Zhang Zuolin of the Fengtian clique based in Manchuria. Zhang Zuolin was considered the most powerful overall; he controlled only 3% of China's population but 90% of its heavy industry, and had the backing of Japan.
Warlords relied on confiscatory taxes, loans from banks, looting, opium sales, and money printing. Feng Yuxiang took in some $20 million per year from opium despite his public anti-opium stance. Some warlords printed currency far beyond their silver reserves; the warlord of Hunan printed 22 million Chinese dollars against a silver reserve of only one million in a single year.
Russian emigres who fled after the Bolshevik victory were widely employed as mercenaries by Chinese warlords. The most feared unit was led by Gen. Konstantin Nechaev, who fought for Zhang Zongchang in Shandong. By 1927, Chinese warlord Sun Chuanfang had reduced Nechaev's brigade from 3,000 men to only a few hundred.
Chiang Kai-shek launched the Northern Expedition in 1926, using the National Revolutionary Army built with Soviet assistance. The expedition destroyed the Zhili and Anhui forces, and Zhang Zuolin was assassinated by the Japanese in 1928. His son Zhang Xueliang accepted Chiang's Nationalist government on the 29th of December 1928, formally reunifying China.
Peasants organized into militant secret societies and village associations that functioned as self-defense militias. Groups like the Red Spear Society went into battle relying on belief in protective magic and invulnerability from bullets. All-female groups also formed, including the Iron Gate Society, which dressed entirely in white and waved fans they believed would deflect gunfire.