A "Union" is an organisation that people who are employees join (become a member of the organisation) so that they can receive support in terms of protecting their rights as workers, increasing their power to bargain for better pay and conditions, and achieving greater job security.
The principle that underpins Unions is that workers have much greater power if they band together rather than if they act individually. An employer generally has enormous power over an individual employee. However, if employees stand together as a collective, they have a level of power that is virtually equal with employees. The ultimate weapon that an employer has is to terminate the employment of workers. The ultimate weapon that a Union has is to call a "Strike", and if this happens all members in the workplace of the Union stop work. Strike are very powerful because they cause employers to lose money and if the strike is prolonged, the business of employer can be severely disadvantaged.
There are many Unions and each one has been set up to look after the interests of workers in a particular industry or occupation. There are unions that look after nurses, retail shop workers, factory workers, teachers, forestry workers, miners and many more types of occupation. A Union accepts individual workers into its membership if they work in an occupation that the union is set up to look after. Unions are mainly national organisations and this means you can join wherever you live in the nation.