Thomas Henson has a taxonomy of non-relational databases:
The first type of NoSQL database is the Columnar databases which is optimized for reading and writing columns of data as opposed to rows of data. Column-oriented storage for database tables is an help drive down the input/output requirements for database. Since the I/O profile is lowered, overall storage footprint is lowered. One main feature of Columnar Databases is their ability to compress data. Instead of data being written in traditional row orientation, Columnar databases use column orientation. Each column will be associated with column key. Checkout this example from my HBase Blog Post.
He then goes on to describe the other three types. I agree with the taxonomy he uses.