An annuity is a series of fixed cash payments made over time. An annuity can be a loan (such as a home mortgage) or an investment (such as a monthly savings plan).
The Rate and NPer arguments must be calculated using payment periods expressed in the same units. For example, if Rate is calculated using months, NPer must also be calculated using months.
For all arguments, cash paid out (such as deposits to savings) is represented by negative numbers; cash received (such as dividend checks) is represented by positive numbers.