Short answers, pulled from the story.
A trade union is an organization of workers whose purpose is to maintain or improve the conditions of their employment. Its aims include better wages and benefits, improved working conditions and safety standards, complaint procedures, rules on promotions and just-cause conditions for termination, and increased bargaining power for workers.
Modern trade unions originated in 18th-century Britain, where the Industrial Revolution drew masses of people into the cities and created a new class of worker. They became popular in many countries during the Industrial Revolution, when employment rather than subsistence farming became the primary way of earning a living.
British trade unions were finally legalized in 1872. This followed a Royal Commission on Trade Unions in 1867, which agreed that the organizations served the advantage of both employers and employees. Earlier, the Combination Act of 1799 had banned trade unions and collective bargaining by British workers.
Trade unions fund their head office and legal team functions through regularly imposed fees called union dues. Union representatives in the workforce are usually workplace volunteers appointed by members through internal democratic elections, and union leaderships are usually formed through democratic elections.
A closed shop employs only people who are already union members, while a union shop hires non-union workers but sets a time limit within which they must join. An open shop requires no union membership at all, and in the United States the Taft-Hartley Act of 1947 outlawed the closed shop.
Trade union density is highest in the Nordic countries. In 2018, Iceland recorded 91.8% and Denmark 66.5%, far above the OECD average, which fell from 35.9% in 1998 to 27.9% in 2018.
The largest trade union federation in the world is the Brussels-based International Trade Union Confederation, created in 2006. It has approximately 309 affiliated organizations in 156 countries and territories, with a combined membership of 166 million.