With real money, you can purchase credits, the currency that is used to purchase in-game items.  Credits can also be earned by watching ads and completing offers. 

With credits, you can buy cases that contain skins for the weapons.  The skins only offer a cosmetic enhancement, they do not change game play in any way.  Cases contain a randomly determined skin, if you obtain a previously owned skin when opening a case, you will receive blue tokens, a currency that can be used to buy skins directly from the load out menu.