In the beginning, World of Warcraft servers were isolated, lost and alone, spinning through the void. Then came a cross-realm dungeon finder, letting players across servers group together to hunt loot, followed by cross-realm raids. Now Blizzard's going the whole hog and introducing cross-realm zones, joining underpopulated areas of the world across severs.
Blizzard explains as most players are around the level cap, lower-level areas of the world are near-deserted for new players. To fix that, underpopulated zones may be shared, letting players from other severs see and adventure with each other. Capital cities and other bustling areas won't be joined, nor will auction houses.
You'll be able to group with players from other servers, chat, and collect boar livers together, though not trade. However, your party won't necessarily all stay in the same instance if you travel to a new zone. Servers will be divided into pools which cross over, so it is likely you'll see the same people some time again, though.
Blizzard's also planning to use this tech to split certain zones into multiple instances per server, such as overpopulated starting areas. You won't be able to see or interact with players in other instances of these areas, or have to fight them for quest goals.