A credit slip is first and foremost a written proof that a product has been returned. Most of the time, the user can use it as a voucher.
A voucher is a discount code which doesn't does not have to be tied to a merchandise return or a refund, and which can take many more forms than a simple credit slip:
You can apply a voucher to all customers,and
or a group of customers, or a single customer; you can set its expiration date;
A cart rule is basically an advanced version of a voucher: in addition to what a voucher could do in PrestaShop 1.4, the cart rules system introduced in PrestaShop 1.5 enables you to: