How do Gyms work?

[Phases]

I'm only level 7. I've never used a gym though, mostly kept to myself with this game. What's the point, benefits? Every one I have seen has stronger Pokemon on it..

 

[Prinny]

I THINK its like...if your team and your pokemon are on the gym you can earn coins.

I tried, and I almost toppled the one here. So this is only my guess...but its a goal for me to take over the one here by work