A contract of indemnity involves two parties, where one party promises to compensate the other for any losses incurred. A contract of guarantee involves three parties - a principal debtor, a surety who guarantees the debt, and a creditor. The key differences are that indemnity provides reimbursement for losses, while guarantee provides security for the creditor. In guarantee, the surety is only secondarily liable if the principal debtor defaults, whereas the indemnifier is primarily liable. A valid guarantee also requires consideration and cannot involve misrepresentation.