This supplements Noah Stein's answer. Judea Pearl has written extensively on causal models, both in his book Causality, and in individual papers, such as "Causal Diagrams for Empirical Research," Biometrika, 82(4), 669-710, 1995, from which this figure is taken:
          Fig1 http://cs.smith.edu/%7Eorourke/MathOverflow/PearlFig1.jpg

This supplements Noah Stein's answer. Judea Pearl has written extensively on causal models, both in his book Causality, and in individual papers, such as "Causal Diagrams for Empirical Research," Biometrika, 82(4), 669-710, 1995, from which this figure is taken: