ETL stands for Extract, Transform, Load. For example, you receive files or other data from vendors or other third parties which you need to manipulate in some way and then insert into your own database. Rhino ETL is an open source C# package that I have used for dozens of production processes quite successfully. By using C# for your ETL tasks you can create testable, reusable components more easily than with tools like SSIS and DTS.
It is good to see more information available on Rhino ETL.