What is the difference between bank interest and Qirad?

The legitimate loan (Qardh Shari') is a contract of benevolence and an act of worship, not intended for profit or gain, and it is guaranteed for the lender. Whereas Qiradh (Mudarabah) is a contract of exchange and partnership, intended for profit and benefit, and its capital is not guaranteed without transgression or negligence. As for the usurious loan (Qardh Ribawi), it is guaranteed capital with specified, guaranteed interest, and it is prohibited because it exploits people's needs.