Both are "money to spend" in the store, but they arise in different ways.
Gift cards
A gift card is bought in advance — the customer (or someone giving a gift) pays for the gift card, which can later be used as payment. Gift cards can have different types and rules.
Credit vouchers
A credit voucher typically arises from a return, where the customer receives the value as store credit instead of money back. It can then be used on a later purchase.
Why the difference is worth knowing
They are handled as payment at the checkout but are posted and tracked separately, because they have different origins. This gives a correct picture of what the store owes in gift cards and credit vouchers not yet redeemed.
Read more in Gift cards and Credit vouchers.