FAQ Q245: What's the difference between coupons and vouchers?

Vouchers are meant to give your customers one-time discounts (for given amount or based on a percentage of the total amount). A voucher code can be used only for a single reservation. Coupons, on the other hand, are purchased INDEPENDENTLY of a reservation and can be used for a number of reservations, until the coupon's credits are used up. Coupons can be directly purchased by the customers (or they can be entered by the admins) whereas vouchers must be entered by the administrators and given to the customer in some way. Coupons can be based on a code or can be applied to given email address whereas vouchers always have a voucher code associated with them.

See Q190 for more info about vouchers.
See Q244 for more info about coupons.