Data driven is the property by which the Informatica Server decides the way the data needs to be treated whenever a mapping contains Update Strategy Transformation.
For example Whenever we use Update Strategy Transformation we need to mention whether its DD_UPDATE or DD_INSERT or DD_DELETE in the mapping.One mapping may contain two or more Update Strategy Transformation. Therefore, in order for the session to get executed successfully mention Data Driven property in the session properties for that particular mapping.
For DD_UPDATE records needs to be updated DD_INSERT records needs to be newly inserted. DD_DELETE records needs to be deleted.