Quality

The Quality of an Item represents its rarity. All non-consumable​ items have one of five possible qualities: Common, Uncommon, Rare, Epic, and Legendary.
The only way to improve an item's quality is via the Synthesis Machine.

Attributes

Higher-quality items have more (or in the case of runes, higher) attributes. The number of additional attributes can be found by assigning each item quality to a number (with Common being 0 and Legendary being 4). This number represents the number of bonus attributes to items. In general:

Higher-quality artifacts have more secondary stats.

Higher-quality runes have higher effect scaling. The scaling depends on the effect.

Higher-quality weapons have more Standard Sockets.

In Crafting

All items (not resources) used in Crafting must be of uncommon quality or higher.
A crafted item's quality depends on the lowest quality of the items used to craft it. For example, using two epic items in a recipe will result in crafting an epic-quality item. There is a chance of crafting a higher quality item. If only resources are used to craft the item, the item defaults to common quality.
Upon a crafting higher-quality item, the resulting item gets bonus effects. Similar to normal bonuses, the number assigned to the quality represents the number of bonus attributes. The bonus effects to crafted items, per profession, is below:

Alchemy

Since Alchemists craft consumable items, crafted items do not have a quality. Instead, the chance of crafting higher-quality consumables is replaced with a chance of crafting 1 or 2 additional consumables.

Construction

In Construction, bonus secondary stats will be added to the resulting artifact. This is in addition to the combined secondary stats from the artifacts used in crafting.

Mysticism

In Mysticism, more effects are added to the resulting rune.

Weaponsmithing

When crafting chips in Weaponsmithing, the resulting chip will have bonus effects.