Longevity research and ageing research are sometimes treated as one of the same, and this is not entirely true.  Longevity research is predominantly focused on investigating factors (such as the over expression of certain genes) which extend life-span (longevity) of a model organism (often fruit flies, worms and mice).  Ageing research is more concerned with biological mechanisms of ageing, such as oxidative stress or cellular senescence for example.  Of course there is overlap between longevity research and ageing research.  In longevity research, biological alterations (or intrinsic factors) which have led to life extension in an organism is probably the result of modulating a mechanism of ageing.  The difficulty is knowing what the altered mechanism may be. Similarly, increased longevity of an organism is often used as a parameter to determine successful modulation of a particular mechanism of ageing.