Have you ever heard about MongoDB? If yes, you might know what a Document-Oriented Database is. In case you don’t know have any idea what I’m talking about, a Document-Oriented Database stores data in a semi-structured fashion while a Relational Database uses a rigid and highly strict structured data storage. The main difference is that all data for the document/object in question is stored in the document/object itself.
Got interested? You’re lucky, Karl Seguin recently created and release an online 15 minutes interactive tutorial in MongoDB. So, if you’re curious about MongoDB but never had the chance to test it, just access this interactive tutorial and learn it’s basics and how a Document-Oriented DB works.
MongoDB (from "humongous") is a scalable, high-performance, open source, document-oriented database. Written in C++ and currently is used by many projects. Take a look at their site for more details.