A union works almost the same way an HOA works. A group of people get together and sign a contract agreeing to abide by certain rules.

Internally, there’s generally a governing board, usually an elected one, but occasionally you get a dictatorship type deal. They may or may not implement rules, they might issue IDs of some sort, they almost invariably hold meetings, and almost always charge dues, which is a fee members pay to be a member, in principle to fund union activities.

The leverage a union holds it’s in its membership. Generally the rules of a union are that as a member you agree to certain actions, and the obvious is to strike. The most common union rule is that if the union body calls a strike, you aren’t allowed to come to work. That gives the union body the ability to force a company to meet certain demands or face the prospect of not having anyone show up.

The basic overall concept is that the union leadership negotiates with company leadership on behalf of the union members.

The downsides are also similar to an HOA, especially when a union gets very large. You can find yourself subject to rules governing your behavior, your appearance, your hours, etc. You might find yourself being more an employee of the union than the company.

Small unions are almost always beneficial, but can lack negotiating power. Large ones can and often do become self serving and overly political, but have a lot of power to affect pay, benefits, hours, etc. for their members