What is ETL, and what is ETL testing?

In ETL, data is extracted from source systems, transformed into a consistent data type, and then loaded into a single repository. During ETL testing, data is validated, verified, and qualified to prevent duplicate records and data loss.

The ETL testing ensures that data is transferred from heterogeneous sources to the central data warehouse in compliance with all transformation rules and validation checks. This is different as opposed to the data reconciliation used in database testing, ETL testing is applied to data warehouse systems to obtain relevant information for analytics and business intelligence.