Visualization is an activity that helps us solve problems better and understand better our problems. It is therefore natural to try to come up with some computer support for our visualization activities, so some researchers have defined information visualization as "the computer-assisted use of visual processing to gain understanding". A more sophisticated and detailed, but essentially similar definition says that information visualization is "the process of transforming data into visual form making use of humans' natural visual capabilities" to make that computer-supported visualization useful for users. But information visualization can be also defined as a purely cognitive activity, that does not require the use of computer, as in the examples I have made before. In this definition, IV is defined as "the process of forming a mental model of data" - mental models are the representations of the world we build in our minds - to gain insight and a deeper understanding of that data. In this case, if we adopt a purely cognitive definition of information visualization, then when we build a computer program to support this activity we call this program an IV tool, an information visualization tool.