Pokemon Go's new Raid Battles are now live! You'll need a Raid Pass which you can get by spinning a photo disc at a gym or you can buy a Premium Raid Pass from the Pokemon Go Shop which costs 100 coins. If you defeat the Raid Boss at the Gym where the Raid Battle is being held, you’ll earn special items and a chance at catching the Raid Boss.
From the Official Pokemon Go website: "You’ll receive a notification of Raid Battles happening nearby. You can also use the Nearby feature to locate raids happening around you. Don't worry if you miss a chance to join a Raid Battle as they happen frequently. Before the raid begins, a Raid Egg will appear on top of the Gym. A countdown will display the time until the Egg hatches and the battle begins."