Items come in Normal, Magic, Rare and Unique rarities, each with different modifier potential. Item level controls which modifiers can appear.
Armour and weapons can have rune sockets; runes add fixed stats such as resistances or maximum life. Skill and support gems are socketed into your skills (not your gear) - supports link to a skill via support sockets added with Jeweller's Orbs (up to 5).
Use currency orbs to craft: Transmutation/Augmentation for Magic, Alchemy/Regal for Rares, Exalted to add mods, Divine to reroll values, and Chaos to swap one mod for another.
Deterministic options include Essences (add a specific mod by tier), Alloys (guaranteed exclusive mods, 0.5.0), and Desecration/Bones crafting from Abyss.
Fracturing Orbs lock a desirable mod in place, and Runeforging with Verisium can add Runic Ward to armour.