What is the difference between interest expense and inrest payable?

Interest expense is an expense (cost) as the name implies. Interest payable is a liability. When you are accruing interest, your entry would be a debit to Interest Expense and a credit to Interest Payable.

And to expand on what JLo is saying, when you actually pay the Interest (Interest Payable), you would debit that account and credit the Cash account.

And to expand on what the other two said...

Remember that an expense is for a current period only and is closed out at the end of the year. A liability is at a specific point in time. The interest payable could be from the current year or a prior year or anything. The balance can go up and down, and the balance is what is a liability at a specific point in time. The expense accumulates during the current period and ends when the current period ends.

This is true of all expenses and all liabilities. (And for that matter, is true of all income statement accounts and of all balance sheet accounts. If you understand how this one works, the same reasoning can be applied to any other account.)