Set bonuses are returning for the first time in 4 years. For players who started in BFA or Shadowlands, this is a new system. Essentially, you will gain a bonus effect unique to your specialization when you equip 2 pieces of set gear and a second bonus effect when you equip 4 pieces of set gear. The set pieces are the Head, Shoulders, Chest, Hands, and Legs slots. “What about my domination sockets?” you may ask - dom sockets and their effects are now only enabled in the maw and the SoD raid. You may safely replace these items.
The Chest and Shoulder set pieces drop at +7 ilvls. Almost all classes will want to use these items no matter their stat priority. The ‘best’ slot for most classes to craft their legendary will be helmet due to higher stats and guaranteed socket, but check your individual class discord for more information. If your main power can’t be crafted in a helmet, you can craft your Unity power in that slot once you obtain it in week 5. Legs are a good backup option but do not come with the guaranteed socket.
Set pieces can be acquired in a number of ways. From raids, bosses 6-10 will drop 1 piece of set gear per 10 people in the raid. During the first week of launch it will not be possible to get a 4-set bonus as bosses 9, 10, and 11 are not available. Set pieces will not drop as items but will instead drop as tokens. The tokens are Mystic (Druid/Hunter/Mage), Venerated (Paladin/Priest/Shaman), Zenith (Monk/Rogue/Warrior), and Dreadful (DH/DK/Warlock). You can only trade a set piece if you already have a set piece of the appropriate ilvl in that armor slot.
An additional source of set pieces is your weekly great vault from PvP, M+, and Raid. The item level of the set piece will be whatever level you earned for the vault. The stats for the set pieces are always the same regardless of what content they came from. There will be no min-maxing your vault items trying to change the secondary stats of the set gear.
Finally, we have information on the set catch up mechanics. 8 weeks into the patch, all players will gain a charge of an item called the Creation Catalyst. This will allow you to use Cosmic Flux to convert a piece of loot into a set piece. The piece's secondary stats will change to those of the set item but it will keep any valor/honor upgrades, sockets, or tertiary stats it might have. These charges will be generated globally at the same rate, so there is no need to worry about logging in immediately to craft a piece. Until you have the Creation Catalyst, you should not pick a non-set item in a set item slot from your weekly vault.
A full list of set bonuses can be found here. The info about Creation Catalyst can be found here.