This thought is not mine, but I cannot recall the source: NoSQL is a bad name; it should be NotSQL. As a result it is a very large umbrella. When one sees NoSQL it is a safe assumption to think MongoDB. But you could also think DB4O (I like this much more, in an abstract way). So you can go about learning any of these technologies since you find an instance of NotSQL. To learn NoSQL, you are really still able to learn this. You are learning a philosophy rather than a technology.
I remember reading something by Erik Meijer where he states he also thinks it is an unfortunate name. I think he suggested coSQL, and gave an interesting perspective on the relationship between SQL and noSQL.
