The transformation of Data Warehouse

A data warehouse is created by transforming an operational database to a data warehouse. The transformation includes a series of steps of data manipulation, such as extracting, cleaning, aggregating, summarising, combining, altering, appending, etc., all of which are called an Extract-Transform-Load or an ETL.

Could someone to explain the Extracting, Cleaning, Aggregating, Summarising, Combining, Altering, and Appending?