Assure, Ensure, Insure

To assure is to promise or say with confidence. It is more about saying than doing: I assure you that you'll be warm enough.

To ensure is to do or have what is necessary for success: These blankets ensure that you'll be warm enough.

To insure is to cover with an insurance policy.

What you insure you entrust to a business. What you ensure results from your personal efforts.