this link will probably explain you the details: http://www.elearnenglishlanguage.com/difficulties/assureensure.html ...and I can assure you of that. :)

juan, do not forget "insure" which means to guarantee against loss or harm. There is a link that helps with when to use insure, ensure and assure: http://www.yourdictionary.com/grammar-rules/Insure_vs_Ensure.html Hope this helps.

"ensure" could also mean make safe or secure, protect.

ensure : Be careful or certain to do something; make certain of something assure : Assure somebody of the truth of something with the intention of giving the listener confidence "I assured him that traveling to Cambodia was safe"

Assure: to promise or say with confidence Example: Let me assure you that I will be at the meeting at noon. Ensure: to make sure something will/won’t happen Example: To ensure my family’s safety, I have installed an alarm system.