Anytime you do not pay your balance in full by the end of the period, you are borrowing money from the issuer of the credit card (usually a bank), at fairly high interest.
Ignoring the issue of ribis, that interest in compounding at a high rate and over a short period of time the compounded interest becomes a problem in itself.
If you pay off your account on time, you can have up to a six week “free loan” from the credit card company (time between buying the goods/services and paying for them).