To "assure" a person of something is to make him or her confident of it. According to Associated Press style, to "ensure" that something happensis to make certain that it does, and to "insure" is to issue aninsurance policy. Other authorities, however, consider "ensure" and "insure" interchangeable. To please conservatives, make the distinction.
However, it is worth noting that in older usage these spellings were not clearly distinguished. European "life assurance" companies take the position that all policyholders are mortal and someone will definitely collect, thus assuring heirs of some income.
American companies tend to go with "insurance" for coverage of life as well as of fire, theft, etc.