Skeletons offer the best chance for fossilization because they exist in the best condition, namely, they are much harder than other organic matter (e.g., tissue). However, despite their hardness, animal skeletons still need to be buried quickly, lest they be devoured by bacteria or dissolved by rain. The type of material that the skeleton is buried in is also important for fossilization: too much pressure from the soil, and the skeleton will be crushed. Sediment in water bodies and clay are both ideal conditions for preserving a skeleton as a fossil.
Skeletal fossils are not always found unaltered (which means they are exactly as they were in the animal after it died); instead, there are several different types of fossilized skeletons. These include permineralized fossils (which have had the spores of bones filled in with inorganic material), recrystallized fossils (which have had the original elements that made up the skeleton re-arranged at the atomic level) and replacement fossils (in which all the original material from the skeleton has been replaced by inorganic matter).
Bones are not the only type of skeleton that animals leave behind; indeed, different types of animals have different types of skeletons. The bones of vertebrates are made up of calcium phosphate, which is white when in the organism but can become black when exposed to heat. The skeletons of invertebrates are not necessarily as long as vertebrate bones; these include the shells of mollusks (which are made of aragonite) and the shells of arthropods (which are made of chitin).
There are also some types of fossils that are not skeletons, but are different remains of organisms. Freezing has the ability to both retain the skeleton and the internal organs of an organism, while molecular fossils are fossils that contain genetic material, but nothing structural like the skeleton. Additionally, there are some types of fossils which have nothing organic in them, but are just preserved remains of the animal's behavior, such as foot prints.