What is ETL after all? ELT? Anyone?

Why is it so hard to find a unique consistent definition of “ETL”?

Let’s start from the beginning. In the old days, most data systems ran on databases, as did the Data Warehouse (DWH) server, and replication was usually handled by the database system. Let’s say a company used SQL Server for the operational databases, then your data warehouse would probably be running on top of SQL Server as well, and you would replicate data into the staging area using live replication, backup/restore, or log shipping.