There are some source systems or an application that lacks the interface needed to extract data for ETL Processing .In that case we cannot directly extract from the source.What we can do is to take copy of the source data and dump into flat file. This file will give a snapshot of the source system and hence is called Snapshot Sources.
Snapshot Source provides the snapshot of the Data at the time of data extraction. We can track the change of data by comparing successive snapshots. This approach is also known as snapshot differential.