What is labor union and what is advantages and disadvantages of it?

There's different pros and cons to labor unions. In some industry's I believe that they are still necessary (nurses union, coal miner's union, writer's union). In other industries (grocery stores), I personally think they're a rip off. Most labor unions require an initial fee and then monthly dues. In return, you get certain benefits (insurance, longer lunches, it's harder for you to be fired, organized strikes, additional retirement). I'm personally totally against grocery store unions because I worked at a union grocery store when I was 16 and they basically wanted me to give them 1/4 of my paycheck (remember, I was 16 and working like 20 hrs max at just above minimum wage) and I wasn't even getting anything in return because I was still on my parents health insurance and the state requires that I get an unpaid lunch and a 10 minute break or every 4 hours (what the union garunteed me). I guess maybe if you're older and have a family to support or something a grocery store union might be worthwhile. And it certainly seems important in other industries.