What is NoSQL, exactly? Aside from the basic implications of the name - I guess it's not SQL, right? - it can be hard to define, and categorizing and comparing different databases is a helpful approach. According to Robin Johnson on the SendGrid blog, NoSQL is four things: document-oriented databases, in-memory databases, graph databases, and column store databases. For each database type, Johnson provides the following examples:
Column Store Databases
It's an interesting look at the NoSQL movement and the categories (and specifics) of the various databases that define it. Check out Johnson's full article for more details.