Gloves/Boots/Helmets/Body armours/Weapons can have rune sockets

Runes provide powerful stats like 12% elemental res or 25 max life

Use Artificer's orbs to add sockets to items

Salvage quality gear for armourers scraps

Salvage socketed gear for artificier's shards (10 for an orb)

First guaranteed 4-link from Orok Campfire chest in Act 3