Fossils are the preserved remains of plants or animals. For such remains to be considered fossils, scientists have decided they have to be over 10,000 years old. There are two main types of fossils, body fossils and trace fossils. Body fossils are the preserved remains of a plant or animal’s body. Trace fossils are the remains of the activity of an animal, such as preserved trackways, footprints, fossilized egg shells, and nests.
When asked what a fossil is, most people think of petrified bones or petrified wood. Permineralization is a process. For bone to be permineralized, the body must first be quickly buried. Second, ground water fills up all the empty spaces in body, even the cells get filled with water. Third, the water slowly dissolves the organic material and leaves minerals behind. By the time permineralization is done, what was once bone is now a rock in the shape of a bone.
When an animal or plant dies, it may fall into mud or soft sand and make an impression or mark in the dirt. The body is then covered by another layer of mud or sand. Over time, the body falls apart and is dissolved. The mud or sand can harden into rock preserving the impression of the body, leaving an animal or plant shaped hole in the rock. This hole is called a mold fossil. If the mold becomes filled over time with other minerals the rock is called a cast fossil. (http://www.kidsdinos.com/what-are-fossils/)
Fossils give us information about how animals and plants lived in the past.
Once people began to recognize that some fossils looked like living animals and plants, they gradually began to understand what they were. They realized they were actually the ancestors of today's plants and animals.
Some fossils are easy to identify and look like plants and animals alive today.
While we can easily recognize and identify some fossils, many fossils represent animals that no longer exist on Earth. We only know about extinct groups like dinosaurs, ammonites and trilobites through fossils.
Some animals and plant are only known to us as fossils.
By studying the fossil record, we can tell how long life has existed on Earth, and how different plants and animals are related to each other. Often we can work out how and where they lived, and use this information to find out about ancient environments. (http://www.oum.ox.ac.uk/thezone/fossils/intro/proof.htm)