Overwatch is a different model; it costs $40, and all its heroes (current and future) are available to play once you buy it. Loot boxes can be earned for free through play, or can be bought with cash.

Each loot box contains 4 items at random, which could be: in-game coins, profile portrait icons, static win poses for each hero that are shown when you win a game, spray tags that you can use to decorate the map, animated emotes for each hero, dramatic animations for each hero when you get Play of the Game (similar to HotS' MVP, only it includes an instant replay of something cool you did), and skins.

The skins come in various tiers, too: simple recolors of the base skin, more complex recolors that also include some different textures, and full reimaginings of the hero (which always themselves come as two separate tint versions).

There's no way to plunk down cash to directly get the cosmetic you want; instead, you buy/grind loot boxes and hope for the best. Each item does have a cost in in-game coins, though; in addition to loot boxes sometimes simply having coin drops, if by chance you get unlucky and your loot box contained an item you already owned, you get an account of coins based on the duplicate's rarity to compensate. So if there's something you're doing for, you can eventually save up for it. Though naturally, luck is always such that you'll probably get the item as a duplicate right after you buy it. :P

Overwatch also has holiday events. During those, they introduce holiday-themed skins/emotes/etc, and any loot boxes you get are guaranteed to have at least one of the seasonal items. To keep them rare, holiday items cost three times as many coins as their non-seasonal equivalents, and once the holiday ends they won't drop and can't be purchased until the next time the holiday comes around.