Rarity

Rarity and Quality are properties of Items, which are represented in-game by the color and name under the item's name and (in most cases) by a title prefixed to the item's name. An item can have only one rarity and quality, which is determined when the item is created and remains unchanged, except in rare circumstances.

Rarity and Quality are purely cosmetic properties, and have no effect on gameplay other than some additional on-screen messages, except in the case of Unusual quality.

Unusual
Unusual is a special prefix rarity, such as Strange, that gives the Couriers it is used on a special particle effect. The chance to unbox one of these couriers from a Treasure Chest is approximately 1%.

Strange
Strange is a quality introduced with the Dota 2 Store. Strange items are virtually all items that are obtained commonly from opening Treasure Chests. All Strange items have the capability of upgrading the rank prefix on their name, based on tracking a count of a specific in-game event, usually the number of players that the item has killed.

Arcana
Arcana is a special rarity added in the March 07, 2013 Patch, this rarity directly changes the appearance of the Hero, their mini-map icon, or their abilities. The only Arcana item so far is Fiery Soul of the Slayer.

Rarity
Items with a higher rarity have a less frequent drop rate than items with a lower rarity. Items are currently classified under one of the six following categories, ordered from most common to least common: Common, Uncommon, Rare, Mythical, Legendary, Immortal, Ancient, and Arcana.