data driven is the instruction fed to informatica server 
wheather to insert/update/delete row whenever using update 
strategy transformation

When u configure a session using update strategy the 
session property data driven instructs informatica server 
to use the instructions coded in mapping to flag the rows 
for insert, update, delete, this is the default option 
selected when using a update strategy transformation in a 
mapping.

Data driven is a process, in which data is 
insert/deleted/updated based on the data. here it is not 
predifed tht data is to insert or delete or update .. it 
will come to knw only when data is proceesed

When ever a mapping contains Update Strategy Transformation
the way data needs to be treated is declared in the session 
property as Data Driven. This is done by mentioning 
DD_UPDATE or DD_INSERT or DD_DELETE in the update strategy 
transformation.
A mapping may contain two Update strategy transformations, 
1 for inserting and another for updating in two different 
pipelines in single mapping. 

Therefore, in order session to get executed successfully, 
mention Data Driven property in the session properties for 
that particular mapping.