• Extract is the process of reading data from a database. In this stage, the data is collected, often from multiple and different types of sources.
  
  
• Transform is the process of converting the extracted data from its previous form into the form it needs to be in so that it can be placed into another database. Transformation occurs by using rules or lookup tables or by combining the data with other data.
  
  
• Load is the process of writing the data into the target database.

How ETL Works

Data from one or more sources is extracted and then copied to the data warehouse. When dealing with large volumes of data and multiple source systems, the data is consolidated. ETL is used to migrate data from one database to another, and is often the specific process required to load data to and from data marts and data warehouses, but is a process that is also used to to large convert (transform) databases from one format or type to another.