What are guarantors?

Getting a loan sometimes doesn’t work out because the bank is not certain that the applicant can afford the instalments. A guarantee helps secure this. But what is a guarantee ? It gives a creditor additional security for an agreed payment: in an emergency, the guarantor pays the amount owed. This can be a private person, but also an insurance company or a bank, for example. What exactly is a guarantee, and what criteria must a guarantor fulfil?