Fossils are the remains or imprints of ancient plants and animals preserved in sedimentary rock. There are three main types of fossils based on how they are formed: actual remains, imprints, and petrifactions. Fossils are useful for studying chronostratigraphy, biostratigraphy, paleogeography, paleoclimate, paleoecology, and organic evolution. They can also help discover new deposits of resources like coal and petroleum. Studying fossils provides data about past life on Earth and insights into environmental conditions over time.