Only available as part of a one time redemption when you buy the Overwatch: Game of the Year Edition.
Loot Boxes contain random items that can be used to customize the appearance of your heroes and personalize the ways they express themselves. These can include items such as Skins, Emotes, Victory Poses, Voice Lines, Sprays, and Highlight Intros, as well as Credits that you can use to acquire in-game cosmetic goodies for your heroes.
Items come in common, rare, epic, and legendary quality levels. Every Loot Box contains at least one rare quality or better item.
If your Loot Box contains an item that is already in your collection, you'll receive Credits instead.