The Raid Bosses egg on top of a gym.
A Raid Battle is a special type of Gym Battle in Pokémon GO. It requires trainers to be at least level 5 (the limit would decrease as time goes on until the limit reached level 5, the same minimum level it requires to interact with gyms) and have a Raid Pass to partake in a raid battle. An egg will appear on top of the Gym with a countdown to mark the Raid Battle. Once the timer reaches zero, the raid boss will hatch and will have a very high Combat Power. Pokémon assigned to the Gym will leave when a raid is going to start at that Gym. There are tiers for the raid battle depending on the CP of the raid boss.
A boss in a Mystic Gym
One will need a raid pass to enter, and they can be obtained from the Gyms once a day. One can choose to join a raid with random trainers, or join a private group. Trainers from different teams can join the raid as well, being the only time when the three teams ally. The battle plays like a normal Gym battle. To defeat the boss, the trainer will need several allies. A raid lasts for an hour. After the Raid Battle is over, the Gym functions as it did before the raid. The raid can be joined infinitely after turning in the Raid Battle pass, even after being defeated.
If the Raid Battle is won, the trainer is awarded with Premier Balls, depending on the results. There is also a chance to catch the raid boss; note that its CP will be decreased, and one can only use Premier Balls to catch it, with Berries to ease catching it. If there are no Premier Balls left, the weakened Raid Battle boss will flee.