A track is any marking or impression made in a substrate by the foot or hand of an animal. Opinions vary on how many tracks constitute a trackway, but for our purposes I will define it as two or more tracks in a sequence. Substrates for track preservation vary considerably but most tracks are formed in unconsolidated sediments and fossil examples show the same preservational bias. Exceptions include crushing of hard substrates by the feet of heavy animals (i.e., dinosaurs that stepped on bivalve shells or bones) or claw marks left in wood.
Going by these definitions, tracks are necessarily made by animals with limbs. Thus, tracks can be made by arthropods, amphibians, reptiles, birds, and mammals. Trackways are first represented in the geologic record by arthropod trace fossils such as Cruziana, which were made by trilobites or trilobite-like arthropods beginning about 550 million years ago. Vertebrate tracks are nearly coincident with the first tetrapodal vertebrates (amphibians) in the Devonian Period and tracks representative of reptilian, avian, and mammalian tracemakers are also common in the geologic record after (or slightly before) the first appearance of these major vertebrate lineages as body fossils.
Tracks made by vertebrates can be classified on the basis of whether they were made by bipedal (two-legged) or quadrupedal (four-legged) animals. Bipedalism typically involves movement from one opposite foot to another (right to left or left to right), whereas quadrupedalism can use any variation of four limbs (two right and two left) moving in harmony. The order of foot placement is determined by the behavior of the animal, depending on whether it is walking, trotting, bounding (hopping), or galloping, as well as which direction it is moving.
Tracking may be the oldest science known to mankind because of its use for determining movement and migration patterns of prey animals by hominids. Some people still study tracking today for their livelihood, particularly the indigenous people of parts of South Africa and Australia.
Further Reading on Tracks:
Halfpenny, J., and Biesiot, E. 1986. A Field Guide to Mammal Tracking in North America. Boulder, Colorado, Johnson Books, 163 p.
Lockley, M., and Hunt, A. P. 1995. Dinosaur Tracks and Other Fossil Footprints of the Western United States. New York, Columbia Univ. Press, 338 p.
Murie, O. J. 1982. A Field Guide to Animal Tracks (2nd Edition). Peterson Field Guide Series, Boston, Houghton Mifflin Company, 375 p.
Dinosaur Tracks as Trace Fossils - link to section in the Dinosaur Trace Fossil page.