ETL stands for Extract Transform and Load
it is the process of using built in components to move heterogeneous data and migrate it into a single source such as a text file from a legacy mainframe to a MS Sql Server Database or OLAP CUbe.
SQL Server's Implementation of ETL is known as DTS (Data Transformation Services)
A very capable data migration and transformation process the sum parts of the actions are compiled into a "Package"
these packages can be scheduled or run on command.
Please search books online for more comprehensive information.