The idea of a labor union came with the industrial revolution. As more and more people started working at a single place, they came to the conclusion that the employer was dependent upon the workers, rather than the other way around. If the workers could organize themselves, they would be able to negotiate for higher wages and better work conditions. A single worker could always be fired, but if they all grouped together into a labor union, they had power to force an employer to negotiate.
To prevent this, labor unions were long forbidden in most countries, often with severe punishments. The agitators were often executed to discourage others. Despite this, labor unions grew, and together with the socialist and communist movements, they made good progress.
If a union and an employer can't reach an agreement, strikes very often follow.
Some countries, such as Sweden have strong, centralized unions, where every type of work has a specific union, which are then gathered in large unions. The largest Swedish union is LO, Landsorganisationen. LO has over 2.1 million members, which is more than a fifth of Swedens population.