Skins are equippable costumes you can add to your character. They come in Common, Rare, and Legendary varieties. When equipped, Skins change your Hero in some way, usually by giving them stat bonuses, and sometimes by replacing a Hero Skill with a different one.
Skins all have their own Level, which you can increase by spending Skin Tokens and Bucks.
Skins can be won from the Skin Crate, or purchased directly with Skin Nanodust.