In simple words, fossils are mineral matter that replaced organic matter from prehistoric animals (like the Trilobite, Ammonite and shell fossils of Orthoceras) and plants (in the form of petrified wood), and retained their shape.
Petrified wood are wooden fossils. Ancient trees found themselves into a propitious environment, their biological material decayed but the shape of the cell walls were preserved, and through millions of years mineral deposits slowly started filling in those spaces and taking the shape of the dead tree. The same process occurs for animal fossils as well.
Marine fossils are ancient marine animals that have died and underwent a fossilization process
Trilobites are extinct marine animals that roamed the ocean's floors by the millions. Nowadays you can find fossilized trilobite sculptures in many different shapes and sizes.
Shell Fossils are fossils from animals with shells, like Ammonites and Orthoceras.
Ammonites are extinct shelled animals. It's easy to find Ammonites of all different sizes, from tiny to ginormous, pyritized Ammonites (Ammonites whose organic material was replaced by Iron Pyrite), Opalized Ammonites that display an iridescent flash and play of color.
Some special types of Ammonite are called Ammolite, which are highly iridescent with really strong and intense colors, mostly found on Canada.