A cladogram is a tree-like diagram showing evolutionary relationships. Any two branch tips sharing the same immediate node are most closely related. All taxa that can be traced directly to one node (that is they are "upstream of a node") are said to be members of a monophyletic group.
A cladogram is a diagram much like a family tree showing the phylogenic tree of different species and demonstrating where they evolved from common ancestors. Once taxonomists based cladograms on physical, easily-observed characteristics; today, they can use more reliable information like genetic and biochemical analysis to determine the relationships between different species.
Each branch on a cladogram is referred to as a "clade" and can have two or more arms. Taxa sharing arms branching from the same clade are referred to as "sister groups" or "sister taxa." Synapomorphies are characteristics shared by the taxa branching from the same clade, but not shared by taxa on other branches – for instance, vertebrates share certain synapomorphies that are not shared by invertebrates.
The two basic principles behind assembly of a cladogram are that the process from ancestor to descendant should be shown as simply as possible in the tree; and the taxonomic categories applied to the branches in a clade should be monophyletic, or cover an ancestor and all its descendants. Cladograms should not be seen as evolutionary fact, but only as a possible path for speciation.
MAKING CLADOGRAMS: Background and Procedures
Phylogeny, Evolution, and Comparative Anatomy