Income Tax return is the forms you file to the government to tell them how much income you've had, how much taxes you have paid, CPP, EI etc.
The Notice of Assessment is the form that the Canada Revenue Agency sends to all taxpayers after processing their returns, that states the amount of taxes to be paid or refunded.
If the Canada Revenue Agency has made any corrections to your income tax form or GST/HST rebate application, the Notice of Assessment explains what changes have been made.
If you disagree with the Notice of Assessment you have received, the first thing to do is to contact the Canada Revenue Agency (CRA).
If you still disagree after discussing the matter with the CRA, you can make a formal objection, as long as you do it within ninety days of the date of the Notice of Assessment.