Cladistics is a relatively new tool for classifying organisms. It was controversial when first developed, but in recent years the controversy has largely been removed as its use became better understood.

For many, Cladistics provides a more useful look at evolutionary relationships than do older methods, as it results in a diagram which resembles a bush rather than using the straight lines that have characterized lineage diagrams in the past. The results of evolution over time are much more closely related to a multi-branching bush than they are to a straight line.

Here are some pertinent definitions, from The Evolution and Extinction of the Dinosaurs David E. Fastovsky and David B. Weishampel, Cambridge University Press, 1996.

Clade. Group of organisms in which all members are more closely related to each other than they are to anything else. All members of a clade share a most recent common ancestor that is itself the most basal member of that clade. Synonymous with "monophyletic group" and "natural group."

Cladistic analysis. Analysis of the ancestor-descendant (evolutionary) relationships among organisms using hierarchies of shared, derived characters.

Cladogram. A hierarchical, branching diagram that shows the distribution of shared, derived characters among selected organisms.

And here is a simplified cladogram showing the placement of Aves (birds) within the larger Theropod clade: (Graphical version) (Text version)