How do the ETL tools work ? |ETL Tools Selection in Data Warehousing

How do the ETL tools work?

The first task is data extraction from internal or external sources. After sending queries to the source system data may go indirectly to the database. However usually there is a need to monitor or gather more information and then go to Staging Area . Some tools extract only new or changed information automatically so we dont have to update it by our own. 
The second task is transformation which is a broad category: 
-transforming data into a stucture wich is required to continue the operation (extracted data has usually a sructure typicall to the source) 
-sorting data 
-connecting or separating 
-checking quality 

The third task is loading into a data warehouse

As you can see the ETL Tools have many other capabilities (next to the main three: extraction , transformation and loading) like for instance sorting , filtering , data profiling , quality control, cleansing , monitoring , synchronization and consolidation.