Short answers, pulled from the story.
The queue was a hairstyle in which the front of the head was shaved above the temples every ten days and the remaining hair was braided into a long tail. It was the traditional hairstyle of the Manchu people, called soncoho in Manchu, and was imposed on all male subjects of the Qing dynasty. The penalty for refusing to wear it was death.
The Qing imposed the queue to demonstrate that Han men had submitted to Manchu rule, and to identify those who refused. The shaved forehead was treated as the primary test of loyalty. Han Chinese had no objection to braiding the back of their hair, since they traditionally wore it long, but strongly objected to shaving the forehead on Confucian grounds.
The Queue Order was a decree issued by regent Dorgon on the 21st of July 1645, commanding all Han men in China to adopt the Manchu queue within ten days or face execution. The order was partly driven by Sun Zhixie, a Han official from Shandong, who had voluntarily shaved his own forehead and demanded that Dorgon impose the style on the entire population.
The violence was severe. Three separate massacres in Jiading within a single month killed tens of thousands, with the third massacre leaving few survivors. At Jiangyin, between 74,000 and 100,000 people died when the city walls fell on the 9th of October 1645. Much of this violence was carried out by Han Chinese defectors who had switched to the Qing side.
The Xinhai Revolution of 1911 ended the Qing dynasty and prompted an almost immediate shift away from the queue. The last Emperor of China, Puyi, cut his own queue in 1922. Some individuals, like Zhang Xun, continued to wear it as a personal tradition after the dynasty fell.
European soldiers wore queues from at least 1678, when a depiction of King Louis XIV of France shows the king hunting with his hair tied back. Grigory Potemkin made the Russian Army the first western military to ban the style through reforms in the 1780s. In the United States Army, the queue was abolished on the 30th of April 1801 by an order from Major General James Wilkinson.