Ensure means to make certain or make sure. For example, ensure (=make sure) that you are paid for the extra work. Insure is used when you pay money to an insurance company to be protected. For example, to insure yourself, a property, a car.
Please ensure that you lock all doors. We ensure that the president will be safe during the parade.
This company will insure my car for only $40 per month. Ensure you remember to insure your mobile phone against theft before travelling abroad.