Affixes

An affix is one modifier on a magic or rare item. Prefixes are things like "+ maximum life" or "% increased damage". Suffixes are things like resistances, speed or critical strike.

How many. A magic item has one prefix, one suffix, or both. A rare has up to three of each. An affix of the same kind appears only once per item.

Tiers. Every affix rolls in tiers. Tier 1 is the best tier that affix has, the same on every item. Higher numbers are weaker tiers, so a low-level item shows high numbers such as "T7". Strong tiers need a higher item level (Item level) and are rarer. The tooltip shows the roll and its tier, for example "T3".

What affixes do

Damage: increased physical, elemental or spell damage, or damage added to attacks.
Defence: maximum life, maximum mana, armour, evasion, energy shield, resistances, physical damage reduction (The three defences).
Speed: attack speed, movement speed, spell cooldown.
Critical strike: chance and damage.
Recovery: life and mana regeneration, leech, on-hit.
Spell affixes: reduced spell cooldown, reduced spell mana cost, increased spell effect, +1 spell area (tiles, top tier only), increased maximum mana %, and spell critical chance. They roll on caster wands, rods and spellbooks; several also roll on helmets, rings and amulets. Attack wands never roll them.
Spell rank: "+1 or +2 to the rank of one kind of spell" (the kind is rolled with the affix and shown in the name, for example "+2 to Rank of Projectile Spells"). It rolls on caster wands, rods, spellbooks, amulets and attack wands, one per item; not on armour or rings (Spell ranks).
Defensive stat: damage taken from mana before life (Mana before life).

Local and global. A weapon's own damage and speed are local: they change the weapon. Most other affixes are global: they add to your whole character, and several sources add together. The usual rule is: all "increased" bonuses are summed, then each "more" bonus multiplies on top. More-multipliers are rare and mostly come from keystones, ascendancies and glyphs.

Resistance caps. Resistances stop helping at 75%. A few effects raise that cap, such as a Warding flask (Damage types and resistances).

Weapon kinds. An item rolls the affixes of its kind. Caster wands and rods roll spell affixes. Attack wands (Ember Switch and its five siblings, Weapons) roll attack affixes such as "Adds fire damage to Attacks", attack speed and on-hit. Melee and bows roll their usual attack lines.

Implicits. The base item type can carry an implicit modifier (for example attack speed on weapons), listed above the affixes. They stay on the item.

Getting the affixes you want. Use currency. See Crafting basics and Essences.