A guarantee is a promise that is made by one person that is legally recognized for a deal that involves any kind of transaction- be it money, goods or services. It is usually written and signed by the parties involved.

Example:
For example when you buy a fridge, they give you a warranty card that ensures the machine will remain in working condition for up to 2 years. This is a kind of guarantee. All damages are covered by the company up to that period. Thus, a guarantee safeguards you against fraud and cheating.