This is my first post done by request. Clearly my power is growing…
The request specifically covered insure and ensure, but I am making an editorial decision to include assure as well, because it has been dragged into the eons-long feud between insure and ensure, and it deserves to have its pain acknowledged.
According to about.com, all three words mean “to make certain or secure,” but they approach the concept in different ways.
Assure is used in reference to people, and generally refers to the act of putting a person’s mind at ease about an issue. E.g. “I assured him that the barricades will keep the zombies out tonight.”
Ensure refers to actions taken to guarantee an outcome, e.g. “I welded extra plates of steel over the barricades to ensure that they will keep the zombies out tonight.”
Insure specifically refers to a financial and contractual arrangement to cover assets or expenses in the event of an injury, accident, or other loss, e.g. “I insured the house in the event the zombies get in and destroy it.”
Paul Brians (who is way better at this than I am) makes an interesting observation about “insurance” in America versus “insurance” in Europe:
European “life assurance” companies take the position that all policy-holders 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.