When shopping at ebay or making a simple phone call, have you ever wondered "these actions are generating several tupples in a Database, going through an ETL process, filling DataMarts, Data Warehouses"... on and on and on! OLAP, MOLAP, ROLAP, DataMining, MDX queries, cubes... It's a world! We even have some buzzwords like "bigdata" and "semantic data models".
Talking about ebay, here's a "relaxed reading" about their data wharehouse:
"The e-commerce giant stores almost 90PB of data about customer transactions and behaviors to support some $3500 of product sales a second."