Carbon film fossils are formed through a process known as carbonization; this process preserves a living thing, typically a leaf, feather, crustacean or fish, as a thin layer of carbon. These fossils are typically made through compression and are generally flat, with a thin layer of carbon showing the fossil's outline (hence the name carbon film).
While compression, or strong downward pressure, is typically responsible for carbon film fossils, this does not tell the whole story of how these lasting impressions are made. In addition to the compression aspect of the formation process, the future fossil's organic material slowly decomposes and changes over time, with processes like polymerization of lipids (fats) and volatilization of basic elements such as hydrogen and nitrogen taking place. Eventually, the bulk of the organic material that was originally part of the leaf, feather or animal dissolves, leaving only a thin, black layer of carbon behind.
Carbonization doesn't have to refer to fossils that were preserved under this exact process. Some paleontologists may refer to any dark, flat fossil as a carbonization, though it may not exactly be a proper example of carbon film. True carbon film fossils earn that name by being composed primarily of carbon, a highly abundant mineral that causes the dark color of these fossils.