Depending on our provider, you might receive a gift card or a voucher. The key differences are:

Gift Cards add funds to your Wallet, while vouchers are a separate payment method altogether. These funds do not get applied to your Uber Wallet.
The Vouchers always get used first as a payment method. If you apply a voucher, this will get used by default, rather than other payment methods you have enabled.
The Vouchers are only supported on Mobile at the moment. The desktop mode currently doesn’t support vouchers.
The Vouchers are applied to your account by following a link. This should be done on mobile. It can be done on a computer if you are already logged into your account, although the balance may not be viewable until you check on mobile.