I first learned about Databases and SQL years ago when relational database and flat files where the only options for data storage. Since then I had heard about NoSQL and document databases but I did not understand them or why anyone would want to use one. After attending a webinar today put on by Pentaho (a system I use at my job for ETL) one of the graphics really clicked for me and everything fell into place. The graphic shows the progression from relational databases to NoSQL database. Sometimes a simple trigger out of left field will clarify a concept. There was also an image showing which database software support which types of database. which is a good reference.