The difference between a Qardh and a Dayn is that Dayn is more comprehensive while Qardh is more specific. Hereunder are their definitions:
Qardh: Loan of Mithli articles (such as money) with the stipulation to return its like in the future.
Dayn: Money etc. that one is liable to pay back to another. For example: the price one has to pay upon purchasing an item or the amount one has to pay upon destroying an item belonging to someone or a loan that has to be repaid.