Represent activities of the animal while alive, rather than part of the dead creature
Preservation of trace fossils is just like other sedimentary structures: must have rapid burial, and preserved by lithification of the rock itself.
Body fossils: can be preserved in a variety of ways.
In general, only organisms with hard parts can be preserved: shells, bones & teeth, wood, etc.
Main hard part mineralogies/biochemistries:
Carbonate (calcite & aragonite): found in coccoliths, some multicellular algae, forams, some sponges, corals, brachiopods, bryozoans, mollusks, trilobites & other calcified arthropods, echinoderms, etc.: very easy for organisms to generate from sea-water.
Silica (normally cryptocrystalline quartz): diatoms, radiolarians, some sponges
Calcium phosphates (including hydroxylapatite): some brachiopods, vertebrate bones & teeth
And various complex organics, such as:
Cellulose: plants, some algae
Spongin: some sponges
But many organisms have NO hard parts, and are only preserved in rare instances.
Most hard parts are not solid material, but porous. Pore space is occupied by organic material in life. Upon death, organic material begins to decay.
Modes of preservation:
Unaltered: simple burial, some weathering. Becomes rarer (for stochastic reasons) further back in fossil record.
Permineralized: very common mode.
Pore space is filled in with ground water: some dissolved minerals precipitate in pores (probably some contribution by bacterial activity).
Common minerals found in permineralized fossils: silica; calcite.
Original hard parts remain, but extra material added to pores.
Different organisms have different potential for fossilization.
Hard parts vs. no hard parts
Single hard parts (e.g., gastropods & cephalopods) vs. two hard parts (e.g., brachiopods & bivalves) vs. many well-connected parts (e.g., arthropods & echinoderms) vs. many parts connected only by soft tissue (e.g., vertebrates)
Microscopic to sediment-sized to immense
Lived in erosive environments (e.g., mountains) vs. depositional environments
Lived in accessible vs. inaccessible environments (e.g., continental shelves vs. oceanic basins)
Plants a special case: different organs (leaves, stems, trunks, fruit, flowers, seeds, pollen, etc.) are only very rarely preserved together. Each part generally given its own species name!
Many fossils were transported by currents betweeen their death and their burial. These are said to be allochthonous ("other [place] buried"). However, some organisms might get buried in place (in situ, to use the Latin phrase), and are said to be authochthonous ("same [place] buried").
Once in awhile there are depositional settings which preserve extremely fine details or soft tissues that are not commonly preserved in fossils. These are known by the German word for "bonanza": Lagerstätten (singular "Lagerstätte"). Lagerstätten give us great insight into the diversity and anatomy of past life.