This describes the problem of having to end your study before all the objects of study have experienced the event in question. For example, not all of the patients will die during the two years of the study. Not all of the customers will leave in the time frame of the market analysts study. Some, perhaps many, will continue long after the end of the study. Survival analysis handles this data problem quite nicely.
Survival analysis focuses on the first factor, longevity.
Key concepts of survival analysis are the survival function and the hazard function. These functions, or curves, tell a lot about the retention of the group you're studying. It plots the percentage of a group of customers who remain after a given number of months since their first purchase or payment. The dropout rate increases for the next several months until half the starting group are left at about 25 months. After that, the dropout rate decreases rather steadily.