First, you must attempt to resolve the issue with the merchant directly if you haven't already. If the merchant doesn't respond to the your attempts to communicate or won't resolve the issue, you can report it to us to dispute the charge.
Most merchants will offer a refund if there was a problem with a purchase, and getting a refund is much faster than going through the chargeback process. If a chargeback is necessary, however, the issuing bank will take over. The bank will ask for details about the transaction and why the customer wants to dispute it. Make sure to provide all supporting documentation supporting the claim (receipts, email threads, screenshots etc.)
If their reason for disputing the charge is approved, the bank will file a chargeback.