Each interface describes a clearly-defined unit of behavior. For example, the IMoaCalculation interface might define methods to Add, Subtract, Multiply, and Divide. IMoaPrinting might define methods such as ChoosePrinter, StartPrinting, or CancelPrinting.
Classes are units of implementation.
In defining a class, the implementor chooses one or more interfaces to implement, and provides method implementations for the method descriptions defined in the associated interface. For example, a SimpleCalculator class would implement the Add, Subtract, Multiply, and Divide methods in the IMoaCalculation interface using basic fixed point math. A PreciseCalculator class would implement the Add, Subtract, Multiply, and Divide methods using much more precise algorithms, and might also choose to implement the IMoaPrinting interface methods. An object is a run-time unit of operation. If you need three calculators, you simply instantiate three SimpleCalculator objects from the class at run-time, and they are used operationally to do the calculation behavior.