ETL (Extract, Transform and Load) processing is used to load a database from an external source. Normally used in the context of data warehouse systems, an ETL process has the three eponymous functions
Extract: data is extracted from the source system. The source could be a database, file, or API exposed by the source system.
Transform: The data from the source system may not be in the same format as the destination. Transformation is the process of changing the data so it will load correctly into the destination.
Load: The data is loaded into the destination, often through some sort of bulk load facility.
ETL processing can be done with hand-built code in SQL or a client-side language, or it can be done with tools specifically designed for the job (usually referred to as 'ETL tools'). Examples of such tools include SQL Server Integration Services and Informatica Powercentre.