These terms have technical meaning in contract law, but they are most often used to label consumer products. In this arena, a guarantee is a promise of satisfaction, and a warranty (note the spelling) is a type of guarantee, namely one that details the the obligations that the seller or manufacturer undertakes in case the product is unsatisfactory. Thus you may see "Unconditional, money back guarantee," which means that if you're not satisfied with what you bought, the seller or manufacturer is supposed to refund your money.
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