In most case's you should be able to spend your vouchers both in store and/or online. However, this does depend on the voucher supplier as they all have different terms and conditions. 

To get a quick overview of this before you select a voucher, view the bottom of the tab to see where/how the voucher can be spent.

If you are happy with where the voucher can be spent, click on the voucher supplier you would like to order from and more vendor specific instructions on how to redeem can be found before you add to your basket.