In many dictionaries (especially USA), "insure" is alternate spelling for both "ensure" and "assure". Ignoring that, in the most proper usage, "ensure" means to guarantee something will happen in the future. "Assure" means to give someone confidence or make something secure. "Insure" means to buy a policy to protect against loss. "Please ensure that no unauthorized persons enter the playground after nightfall". "I assure you that we will take all necessary precautions". "It is a good idea to insure your home in the event of fire damage".