Effectively your body "Liquidises". As decay sets in, and bacterial action takes place, the cell walls disintegrate and allows the cytoplasm to escape. (a cell is a bit like a water balloon, you can build a relatively solid wall with water balloons- that's how you are 70% water.). In most cases this liquid slowly seeps from the coffin and into the soil.
In some cases though, like older lead caskets, this liquid, called "corpse liquor" can remain sloshing around in the bottom of the casket (It's a highly dangerous "soup" that allows bacteria to thrive and is treated as a bio-hazard as it possibly contains diseases like T.B., Hepatitis, etc.). There is also the action of insects and scavengers (like rats). Corpse flies can apparently burrow up to 6 feet in loose soil to find a corpse (they arrive at a body within minutes of death and start laying eggs in the mouth, nose, anus, ears and eyes).

Not all corpses rot totally down to skeletal remains, there are circumstances where hair and fingernails can remain and even in some cases, soft tissue. Sometimes in an airtight casket or environment, there is not enough oxygen to allow the bacteria to start attacking the cell walls. Or the corpse liquor drains away quickly, taking the bacteria with it, leaving harder tissue like hair and finger nails.