Dictionary.net

Ensure and insure, in simple sense of making sure, were formerly spelled indifferently, either way; they now present an example of differentiation of form when a new idea is developed rendering such distinction desirable. To ensure continues to signify simply to make sure; as, a farmer ensures a good crop by careful husbandry, whereas insure refers to the payment of money in consideration of a certain sum being paid to one's representatives at death, or to secure an indemnity against losses by fire or otherwise; thus a man insures his life or his house. Assure is generally applied to a person, and means to make sure of the truth of a statement; to make certain of something; as, I assure you, i.e. I make you sure, I tell you for certain: it is also used of life insurance in Great Britain, but not in the United States.