This update alters Gym Battles and adds new Raid Battle features.
A new big update is coming to Pokémon GO which will introduce new features and alters Gym Battles as we know them.
First, Gyms will now give items, just like PokéStops and now have six different slots for each Gym. Each Pokémon has to be a unique species. The Pokémon int he gym now have motivation and as it wins or loses, its CP will temporarily decrease. If all motivation is lost, it returns to the trainer. There are now also GO Badges which get leveled up by battling, spinning the Gym location disc, and giving berries to Pokémon in the gym.
Second, Raid Battles have also been announced. These cause a Gym to lose all its Pokémon and all nearby players can challenge it. If successful, then you have a chance to capture a strong Pokémon.
Lastly, new items have also been confirmed and are only obtainable by beating a Raid Battle Boss. A Rare Candy is a mysterious candy that, when used on a Pokémon, turns into that Pokémon’s Candy. Golden Razz Berries will greatly increase your chances of catching a Pokémon you encounter in the wild and can also be given to a Pokémon assigned to a Gym to fully recover its motivation meter. Technical Machines are items you can use to permanently teach a Pokémon a new Fast Attack or Charged Attack.
Source: Pokemon GO Blog