Credits

Credits are loans you can issue to countries or rival banks. Usury is a sin, but bankers don't have souls, so that shouldn't be a problem.

Countries
Loans are essentially low-risk investments. You lend money out of your treasury to a given country, which they then return after a set amount of turns with interest. There are two outcomes: Given that this is a win-win situation, loans have relatively low yields, but offer stable income. Loans are also capped depending on the size of your department.
 * Either they pay up with interest...
 * ...or they default and you send guards to rough them up and either make them pay or seize property to cover the debt.

Banks
With the first upgrade you gain the ability to lend money to rival banks. This is where things get interesting: While loans for countries are capped, the credit you issue to rival banks is unlimited, so you can get a lot more cash out of a loan paid back in full. The interest rates are meager, however...

You can bankrupt enemy banks with loans. If they cannot pay it back, they're kicked out of the game. Issuing a loan to a bank, waiting a turn, then proceeding to wreck them - freezing assets through espionage, waging wars, revolutions, basically doing everything to hurt them and their net worth can eliminate their ability to pay you back. Of course, if they do, this gamble backfires, as you have aided the enemy bank in advancing their position.