All animals with teeth have the capability of leaving tooth marks on any solid substance that they bite, whether these substances are plants, animals, or (who knows why) rocks. A tooth mark is the impression left by a biting animal with teeth, regardless of what was being bitten. Dinosaurs who fed on other dinosaurs left distinctive tooth marks on bones, indicating feeding habits for some types of dinosaurs. In some cases, these tooth marks perfectly match the "dental records" of teeth from known dinosaurs and are preserved in bones of an identifiable species of dinosaur. An example of this direct correlation between "dinner" and "diner" is Tyrannosaurus tooth marks in Triceratops bones. However, such tooth marks do not necessarily mean that Tyrannosaurus preyed upon and killed Triceratops, because the specimen of Triceratops may have been already dead when munched by Tyrannosaurus (thus leaving open the possibility of scavenging behavior, rather than predation).
This tail vertebra from Apatosaurus shows a few tooth marks on the left transverse process, indicating that some carnivorous dinosaur (probably Allosaurus) fed on Apatosaurus. Specimen is in the Museum of Western Colorado's Dinosaur Valley, Grand Junction, Colorado.
This is a close-up (although not too clear) of the previously mentioned toothmarks on the Apatosaurus vertebra.
The end of this limb bone (looks like a tibia to me) from Apatosaurus has parallel toothmarks on it. The spacing of the tooth marks, as well as the individual marks themselves, help to identify what dinosaur was feeding on this apatosaur.
Bite marks that show healing have been discovered in some dinosaurs, and in one instance (Tyrannosaurus again) can be attributed to intraspecific competition. In other words, a healed tooth mark on a specimen of Tyrannosaurus was inflicted by another individual Tyrannosaurus, indicating that two individuals of the same species were fighting with one another. Thus, dinosaur toothmarks can indicate feeding habits and other interactions between or within species.
Further Reading on Dinosaur Tooth Marks:
Erickson, G. M., and Olson, K. H. 1996. Bite marks attributable to Tyrannosaurus rex: Preliminary descriptions and implications. Journal of Vertebrate Paleontology, 16:175-178.