What are they?

Simply put, a union is a group of people that have banded together to affect their working conditions (wages, benefits, environment, etc) by presenting a united front to employers.

Why are they important?

Employers hold vast amounts of power within their organizations while individual employees have little or none. Through collective bargaining, unions provide leverage in negotiations between employers and otherwise powerless employees.

What will employers say to prevent unionization?

Employers will tell you anything allowed by law (and sometimes cross that line) to convince you that it's in your best interest not to join a union.