What is a gym in Pokemon Go?

A Pokemon Go gym is a place where gamers pit their Pokemon against each other in combat. You don’t battle head-to-head. Rather, someone claims the gym and assigns a Pokemon to it. Over time, other gamers will challenge this person’s Pokemon in a fight for control. Whoever wins claims the gym for one of the three teams (Team Instinct, Team Mystic, Team Valor) in the game. Then fellow team members can assign one of their Pokemon to the gym to make it stronger.

Assign one of your Pokemon to a gym to increase the gym’s prestige and level, and lessen the chance of rival players taking it from you.

If you battle players from another team and win, you will lower their gym’s prestige and level.