Two terms come into play when choosing a data storage engine for business. OLAP (Online Analytical Processing) generally concerns analysis of data to support business decisions -- comparing yearly profits, building cross tabs or aggregating customer demographics. OLTP (Online Transaction Processing) generally concerns the daily process of business -- shopping carts, inventory and the general ledger.
You want to pick a database which can support the volume of data you expect, retrieve it in a timely fashion and present it in a usable form. Choose the database that is best able to handle those three needs.
This was first published in November 2006