Bungie just dropped the latest Destiny 2, and it's a wild one. The Iron Banner event was also in the original game, and it allows players to unlock special gear by earning tokens. Right now, you can access the event by going to Lord Saladin in the Tower, which means you'll also have needed to beat the campaign before you can jump in to this event. After you speak to him, Iron Banner appears as its own playlist in the Crucible.
In the original Iron Banner, there were some key differences from this new event. Many of these changes have to do with the fact that Crucible itself has changed, such as the fact that matches are now 4v4 rather than 6v6. Power levels also have no impact on gameplay (like Light levels did in Destiny 1).
The event runs until the weekly reset on October 17th. It's also worth mentioning that a separate event, the Leviathan Prestige Raid, was actually on track to drop today as well. This event has been delayed until October 18th to allow the team at Bungie to fix some last minute exploits.