The graphical table visualization is a bit like a cross table, in that it shows aggregated data, but it can show it in a graphical form. Alongside regular table columns, you can add sparklines, calculated values, conditional icons, and bullet graphs.
Graphical tables are unusual in the Spotfire world; KPI charts and graphical tables are the only visualizations that support direct actions when you click on them. You can set up actions that are triggered when a user clicks on an element in a graphical table. Those actions could be to apply a bookmark, navigate to a page, execute a data function to perform some statistical analysis, or automate Spotfire with IronPython.
That being said, I don't really recommend the use of graphical tables unless you have a specific need for automating actions from a visualization, producing bullet graphs or icons, or want to...