Interest rate refers to the amount charged by a lender. When you borrow money from a bank or other lender, interest is the primary method by which the lender earns income. It's the amount you pay back on top of what you borrow and is calculated as a percentage of what you owe.
Tax brackets are the different ranges of income-assigned tax rates. In the United States, there are seven different tax brackets, with tax rates ranging from 10% to 37%.