See this:
http://dictionary.reference.com/help/faq/language/d51.html
A warranty is a promise or guarantee given. A warranty is usually a written guarantee for a product and declares the maker's responsibility to repair or replace a defective product or its parts. A guarantee is an agreement assuming responsibility to perform, execute, or complete something and offering security for that agreement. It is a promise or an assurance, especially one given in writing, that attests to the quality or durability of a product or service, or a pledge that something will be performed in a specified manner. A guaranty is an undertaking or promise that is the answer to or payment for a debt or default. A guaranty is also something given or held as security until a debt is paid or the performance of a duty is fulfilled. By the way, warrantee is the person to whom a warranty is made and guarantee is one who makes a guaranty.