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Body Fossils
As the name implies, body fossils consist of actual body parts, ranging from teeth and bones to skin and internal organs. Rarely are soft tissues preserved; however, in some circumstances, skin, and even hair, have been discovered. An example is the woolly mammoths discovered in arctic regions. Body fossils of smaller insects are often preserved is sap or amber.
Petrified Remains
A petrified fossil shows the form of a once-living plant or animal "as-is" with one significant difference -- the fossil you see actually consists of minerals, as opposed to the actual parts of the animal or plant. Petrification is caused by a delay in decomposition, usually occurring because the animal or plant became buried under sediment. Water from within the Earth gradually replaces the animal's or plant's form with minerals.
Trace Fossils
Some fossils aren't perfectly preserved and instead only offer clues as to the type of life that created them. Trace fossils -- also called ichnofossils -- come in many varieties, ranging from preserved nests and tracks to excrement. Molds are another common type of trace fossil. Sometimes, when sediment buries plant or animal matter, rather than petrification occurring, a mold forms.
Types of Fossils
Many fossils occur in sandstone -- which forms out of rock. Sediment hardens into sandstone, trapping and preserving any remains. Limestone -- a harder substance than sandstone -- also contains fossils and is a common type of rock close to quarries. Shale rock is also rich in fossils. Formed out of clay, shale hardens and eventually breaks into sheets.