Cladograms are the graphical representation of the study of cladistics. Cladistics is a method of biological classification based on shared characteristics and ancestry. A cladogram contains different species that are linked together via an array of branched lines. The branch points indicate points of ancestry with the species attached to it sharing a common characteristic. So the cladogram is a way to visualize shared characteristics among species and see how far back these characteristics go back on the evolutionary scale. Please note that these ancestors are not necessarily actual organisms, but they represent a branch point in evolution. Some scientists argue that cladistics is no longer a relevant method for biological classification in the modern world with genetic footprinting available.