According to the NetGenesis (SPSS) Glossary of terms:

"Churn measures how much of your customer base 'rolls over' during a given period of time. To calculate churn, divide the number of customers who attrite during the given time period by the total number of customers at the end of the time period."

You can then see how important it is to track the churn-rate and if it is becoming too great then you must do something about it or else.