Here's one definition I found:

A labor union is a group of two or more employees who join together to advance common interests such as wages, benefits, schedules and other employment terms and conditions.

There are other definitions and some countries have certain regulations regarding how a union operates. But I'm curious about people's thoughts on whether they are pro-union or anti-union.

Here are some pros and cons listed by Investopedia:

Labor unions benefit their members by negotiating better pay, benefits, and working conditions. Critics of labor unions maintain that union contracts make it more difficult for a company to fire unproductive employees, and that they increase long-term costs which decreases competitiveness.