Personally I have always understood usury this way.
It is sinful to take interest on a loan when the loan is given to assist a person in a time of struggle as they take care of basic needs.
It is not usury to take interest on a loan when the loan is given to assist a person in accumulating wealth, then a fair share of the accumulated wealth is expected.
In other words John Smith asks Tom Doe for $100, because his car needs to be repaired. Tom Doe should loan this money freely and John Smith should pay it back timely.
However if John Smith asks Tom Doe for $100 with which he plans to purchase widgits that he will then sell for a profit, then Tom Doe can expect a "reasonable" return on his investment and John Smith should pay back the loan timely with interest.
Interesting article from Catholic Workers regarding the Pope's statement to them.
http://www.cjd.org/paper/usury.html
[size=+2][/size]
[size=+2]JOHN PAUL II CALLS FOR END TO USURY: Support for Peter Maurin, Catholic Worker Theme[/size]
On April 14, 1999, Pope John Paul II addressed the members of the National Council of Anti-Usury Foundations and their regional delegations.
The Holy Father gave a special welcome to about one thousand volunteers "who came to call the public's attention to the worrisome and, unfor-tunately, widespread phenomena of usury, which often brings with it dramatic social consequences."
The Pope continued, "I know well, dear friends, the difficulties that you face. But I know that you are determined and united in fighting this serious social evil. Continue to combat usury, giving hope to individuals and families who are its victims. The Pope encourages you to pursue your generous work to build a more just society, one of solidarity, and more attentive to the demands of the needy."
In 1997 Bishop Tarcisio Bertone, secretary of the Vatican Congretation for the Doctrine of the Faith had said that "It seems opportune to publish a new encyclical on the subject of usury and, on the use of money in general," and that this document should be proposed energetically both to people involved in pastoral activity as well as to those in economic endeavors. Bishop Bertone not only denounced the critical aspects of usury, but also "the problem of loans among nations which ends up by creating the problem of international debt."
To Peter Maurin and Dorothy Day, halting usury was a very important step in establishing a just society.
Peter Maurin wrote an easy essay on the subject:
[size=+2]Legalized Usury[/size]
Because John Calvin legalized
money lending at interest
the State has legalized
money-lending at interest.
Because the State has legalized
money-lending at interest
home-owners have mortgaged their homes..
Because the State has legalized
money-lending at interest,
farmers have mortgaged their farms.
Because the State has legalized
money-lending at interest,
institutions have mortgaged
their buildings.
Because the State has legalized
money-lending at interst,
Congregations have
mortgaged their churches.
Because the State has legalized
Money-lending at interest,
cities, counties, States,
and the Federal Government
have mortgaged themselves
in all kinds of financial difficulties
because the State has legalized
money-lending at interest.
Houston Catholic Worker, Vol. XIX, No. 3, May-June 1999.