Discount code: money off straight away
A discount code is deducted at the checkout. The problem is that codes are often expired, apply only to selected items, or do not exist for the store you are shopping at.
Cashback: money back afterwards
Cashback gives you a share back after the purchase, whatever you buy in the store. No code to hunt for, nothing that expires.
Best of all: use both
If the store has a valid discount code, use it at the checkout. Click through via Sparebørsen at the same time for cashback on top. Then you get money off now and money back later, on the same purchase.