There are a lot of software visualization tools existing for various purposes. Therefore, how to choose the right visualization tool for a specific task becomes an important issue. Although there are a few taxonomies for classifying visualization tools, each has its own defects. This paper proposes a new classification schema based on the widely used Bloom's cognitive taxonomy. We summarize a set of questions for each level of the Bloom taxonomy using the well defined verbs, to identify if the visualization tool belongs to the individual cognitive level, which in turn can determine the purposes of the tool. We also conduct case studies on four tools to evaluate this new approach. The result demonstrates that the new classification schema can provide good guidance for people to choose a useful visualization tool.