Bava Metzia 70 - When Interest is Allowed
One is not allowed to put money into a business venture where he shares in profits but not in losses, or even where his proportionate share of profits is greater than that of his losses. The guaranteed part of the investment is then a loan, and the profit on it is a form of interest prohibited by the Sages.
However, one is allowed to invest the money of orphans where they stand to gain more than to lose, by a court appointee.
One is allowed to loan on interest to a non-Jew. In fact, there is nothing wrong with charging interest, but the Torah wants all Jews to behave like members of a family.