ETL / ELT

ETL and ELT are two patterns for moving data: both extract from a source, transform, and load into a destination, but they differ in whether the transform happens before or after the load.

In ETL the data is transformed and cleaned first and then loaded into the warehouse, so what is stored is already finished; in ELT the raw data is loaded into the warehouse first and the transform happens later, inside the warehouse itself, which has become more common with today's powerful cloud warehouses. The difference is not just letter order but an architectural decision: ELT gives more flexibility because the raw data stays untouched and can be transformed differently later, but it needs a strong warehouse and orderly governance, or the warehouse fills with unmodeled raw data. For the consumer of an analytics tool this distinction is usually behind the scenes; what matters is knowing that between the raw event and the number they see lies a transform stage where definitions and rules are applied.