The ‘T’ is ETL stands for transformation. The goal of transformation is to convert raw input data to an OLAP-friendly data model. This is also known as dimensional modeling. Understanding dimensional modeling is essential for building BI systems.
An ETL process is a program that periodically runs on a server and orchestrates the refresh of the data in the BI system. SQL Server Integration Services (SSIS) is a development tool and runtime that is optimized for building ETL processes. SSIS lets you design ETL processes in a graphical way (but if needed you can write parts using VB or C#). The SSIS components are highly optimized for ETL type tasks and the SSIS run-time executes independent tasks in parallel where possible.