What Is A Fossil?

The dictionary defines what a fossil is as: any remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint, etc.
What is a fossil you ask?  Let's take the above dictionary definition a step further. A fossil is a once living organism or impression that over time was replaced by minerals and is basically now a rock.  That's the simplest way to explain what a fossil is.  This is what we call a "petrified fossil".
A very rare (Aspiration Plate Fossil) of a knitia fish from the green River eating another fish. This is an example of a Petrified Fossil.
What is a fossil?  Here's another type of fossil we call a "trace fossil".  This can include a dinosaur footprint for example, tracks, burrows etc.
Trace Fossil of a Dinosaur Footprint.
What is a fossil?  A "cast fossil" is when minerals 'fill-in' the space in a mold.
Cast fossil
Cast Fossil of an Ammonite.
What is a fossil? A "mold fossil" is the space in a rock that has the actual shape of the remains of a living thing.
The left ammonite is the "mold" and the right half of the ammonite is the "cast" fossil.  Dinosaur footprints can also be classified as mold fossils.
Wanna know what else is a fossil? 
And of course the students favorite everytime we show this in schools or at one of our exhibits is the "Coprolite" or Dinosaur Dung Fossil.  Coprolites are the petrified remains of animal dung.
What is a fossil?  here is an example of a Dinosaur Dung called Coprolite.
Just as important, I want you to understand what is NOT a Fossil? Fossils are not actually pieces of dead animals and plants. They are only the impression or cast of the original living thing. The actual living parts decay away but their shape is permanently recorded in the rock as it hardens.
Here I am (Neil Brown - owner of DINOSAURS ROCK) and we've been teaching students and adults for over 14-years now the answer too "What is a Fossil?" I'm pictured above over 6000-ft up in the mountains breaking open shale rock that used to be the sea floor.
What am I hunting for? Trilobite fossils that are over 500,000,000 (thats Five Hundred Million) Years Old.
How in the world did a once living organism become a fossil? Below is a great diagram I got off the internet that clearly explains what a fossil is. Specifically, how a dinosuar fossil forms:
Another graphical example below explains what a fossil is very clearly as well.
1. The animal dies and fall to bottom of ocean.

2. It is covered very quickly with sand and mud to protect it from scavengers. Most of the body parts except the skeleton are eaten by bacteria and have dissloved away.

3. Lot of layers of sand and dirt pile up on top of the skeleton. Water is rich is minerals and the skeleton soaks up the water. The minerals attach itslef to the bone and relplace the actual bone.
4. Erosion and uplift eventually brings the fossil to the surface where it can be found.
 foss animation
• Sedimentary rocks start out soft and squishy ("unconsolidated").
• Fossils form when animals or plants die in the unconsolidated sediments and are covered by more layers.
• Sediments can become hard over time if exposed to higher temperatures and pressures or certain minerals that cement the grains together.
• Fossils are found in sedimentary rocks, but almost never in other rock types. (You need to start out with a soft material to make an imprint. Igneous rocks are very hot when they are soft/molten, so they burn up organic material. Metamorphic rocks are exposed to such intense heat and pressure that any fossils are destroyed).