An interest charge is the sum of interest on your credit card account. It is broken down by transaction type: Purchases, Cash Advances and Balance Transfers. If you pay less than the full balance, pay after the payment due date or if your credit card does not have a grace period for Purchases, then you will pay interest on those transactions.
Please note: Cash Advances and Balance Transfers have no grace period, which means they start accruing interest as soon as the transaction is made. Even if your Cash Advance or Balance Transfer balance is paid in full, you will have interest due (unless you have a 0% promotional rate for these transaction types).