What are gyms and how do I compete in them?

Gyms in Pokemon GO are special locations where players battle to gain control and prestige. To compete in gyms, first reach level 5. Then, find a gym near you on the map and approach it. After selecting a team (Valor, Mystic, or Instinct), you'll be able to battle Pokemon defending the gym. Lower the gym's prestige by winning battles, while aiming to reduce it to zero. Once the gym's prestige reaches zero, it becomes "unclaimed" and you can place your Pokemon to defend it. As a team member, you can also help protect and level up your team's gyms by battling and increasing prestige. Remember to assign a variety of strong Pokemon to defend your gym, and expect rivals to challenge and potentially reclaim your gym.