Quality

In gaming, Quality is what determines, usually, how valuable an item is to a player, though this is not applicable for all circumstances.

Poor
Often called trash or grey items, these are essentially junk items that can be sold to increase a player's currency reserves. They usually have no significant use in-game.

Common
Most items would fall within this bracket; these are for mundane and easily-sourced items.

Uncommon
Typically green in appearance (though not always the case) these are superior-quality to common items, and will usually have superior stats.

Rare
Typically blue in appearance (though not always the case) these are superior-quality to uncommon items, and will usually have superior stats.

Epic
Typically purple in appearance (though not always the case) these are superior-quality to rare items, and will usually have superior stats. In general, Epic items represent the end of the "normal" gearing system, and most replacement items will be epic-quality.

Legendary
Typically orange in appearance, these are items of truly superior status, sometimes only being obtainable by players who have put in significant time and resources into finding them.