The term fossil comes from the Latin word for "dug up," and originally referred to anything dug out of the earth. Today the term is used more specifically to refer to the remains or other evidence of ancient life forms. Typical fossils include ancient bones, teeth, shells, petrified wood, insects in amber, and footprints in rock. Generally, if the remains are more than about ten thousand years old, or at least partially petrified (turned to stone), they are considered fossils. Many creatures found as fossils are extinct; others are not. Most fossils are between 550 million and a few million years old.

Although most types of fossils are large enough to see with the naked eye, others are so small that a magnifying glass or microscope is needed. These are called microfossils, and include a variety of spores, pollen, microorganisms, and fossil fragments.

Note that a fossil must comprose or record some part of an ancient organism. Tools and weapons made by ancient humans are considered artifacts rather than fossils, and belong to the field of archaeology rather than paleontology. Likewise, ancient ripple marks and mud cracks in stone are not fossils, but geologic features.

It will be useful to briefly review the names of several scientific fields that deal directly or indirectly with fossils. Paleontology (the study of fossils and ancient life) is generally considered a branch of geology (the study of the earth), but overlaps with biology (the study of living things). Specialties within paleontology include paleobotany (the study of fossil plants), vertebrate paleontology (the study of ancient animals with backbones, which includes fish, amphibians, reptiles, birds, and mammals), invertebrate paleontology (the study of ancient animals without backbones), and ichnology (the study of fossilized tracks and traces).

Other scientific disciplines related to paleontology include paleoecology (the study of ancient environments), stratigraphy (the correlation of rock layers), taphonomy (the study of how organisms become buried and fossilized), anatomy (the study of the physical structures of plants and animals), and taxonomy, the system by which fossil (and living) organisms are classified. Some of these disciplines will be discussed further in later sections.