The following are common methods you will want to call in your front end templates:
craft.giftVoucher.vouchers()#See Voucher Queries
craft.giftVoucher.codes()#See Code Queries
craft.giftVoucher.getVoucherTypes()#Returns all Voucher Types available.
craft.giftVoucher.getVoucherCodes()#Returns any currently-applied vouchers. This occurs when a customer applies a voucher to their cart.
craft.giftVoucher.isVoucher(lineItem)#Returns whether a provided Line Item object is a gift voucher or not.
craft.giftVoucher.getPdfUrl(lineItem)#Returns a URL to the PDF for this gift voucher, for the provided Line Item object. This will only show vouchers for this line item.
craft.giftVoucher.getOrderPdfUrl(order)#Returns a URL to the PDF for this gift voucher, for the provided Order object. This will show vouchers for the entire order.