IT systems can be broadly classified into OLTP (On-line Transaction Processing) and OLAP (On-line Analytical Processing) environments. ETL tools are predominantly used to move the data from OLTP to an OLAP system. Why do we need to move the data between them?
Namit Kabra blogs about the finer details of these systems with a good number of examples. Read the blog post to get an overview of OLTP and OLAP and the key differences.