Vacuum is a library for extracting graph representations of values from the GHC heap at runtime. Those graphs may then be further processed or translated to various representations for visualization -- like Graphviz, or Ubigraph. By itself this library is not of much use to most users, unless you want to use the raw graph data to visualize the heap in some (new) way. Most people will want to translate the graph representation to something like GraphViz format, which can then be rendered to a pretty PNG or SVG file for viewing. See the vacuum-graphviz package for more.