An ETL tool extracts the data from all these heterogeneous data sources, transforms the data (like applying calculations, joining fields, keys, removing incorrect data fields, etc.), and loads it into a Data Warehouse.
ETL is commonly associated with Data Warehousing projects but there in reality any form of bulk data movement from a source to a target can be considered ETL. Large enterprises often have a need to move application data from one source to another for data integration or data migration purposes. ETL testing is a data centric testing process to validate that the data has been tranformed and loaded into the target as expected.