Robes have no inherent armor bonus, maximum dexterity bonus, skill penalties or arcane failures. They also do not require any proficiency to wear without attack penalties. This makes them the choice not only for spell casters but for Monks, who cannot wear anything other than robes or outfits, the martial-arts clothing of Monks.
From a gameplay point of view, robes are considered armor. You cannot wear both a robe and an armor suit simultaneously.
Unlike armor, robes (and outfits) can be donned instantly, with no wait time to equip them.
Basic robes come in a wide variety of colours and patterns.