What is ETL?
ETL is the process of extracting data from source systems, transforming it into a clean, consistent shape, and loading it into a destination such as a warehouse.
ETL explained
Extract pulls the data from where it lives (a database, an API, a spreadsheet). Transform fixes it up: converts types, standardises names, removes duplicates, joins reference data and calculates fields. Load writes the result into the destination tables.
In classic ETL the transformation happens before loading, on a separate processing server. Modern cloud warehouses often reverse the last two steps, which is called ELT.
Example
Each night a job extracts the day's orders from a shop database, converts all currencies to GBP, removes test orders and loads the result into a reporting table.
How it differs from similar terms
In ELT the raw data is loaded first and transformed inside the warehouse.