How it all works
(or does not work)

When you flush a toilet, drain a bath or sink, cycle a washer or release water in any way into your home's drain system, this water ultimately winds up in your septic system.  When your septic tank fills it starts to function (provided it has it's baffles in place).  If we could see a cross section of a properly functioning septic tank, we would see three distinct levels in that tank.  The first or lower level will be refferred to as sludge.  Sludge is human waste, degraded toilet paper, soap, etc. all of this matter has been proccessed and has fallen to the bottom and is ready to be pumped out.  The next or middle level is called black water which is self explanatory.  The third or top level is called scum which floats on top and is the "new arrivals" .  Scum is basically everything that has passed through the drain system other that H2O. The scum level will reach a level in the tank that is even with the outlet pipe.  At this time this filtered  water starts to gravity flow into the drain field where it will be filtered more extensively and will flow into the soil and ultimately  into the water table.  This is the perfect septic world.

A not so perfect world exists  when one or more conditions  are present to disrupt this process.

  If your outlet baffle is missing or deteriorated, every bit of scum  will flow directly into the drain field
or,
If you have neglected cleaning your septic tank and allowed the level of sludge to rise in your tank,  it will pass under your perfectly sound  outlet baffle and dose your drainfield with  sludge  (which is worse than scum) every time you flush or wash.
or,
Your tank allows ground water to enter it through a seal or crack and just overwhelms your drain field.
or,
You have been using Anti-Bacterial Soaps.  These soaps that contain anti-bacterial agents are a death sentence for your septic system.  Bacteria is crucial to the function of the septic system.  It is a known fact that you must do a 4-5 minute surgical scrub to get any personal hygenic benefit from these soaps.  Now I ask, do you spend 5 minutes scrubbing your hands?  Probably not, but when these chemicals get into your septic tank they put a hurtin on your bacteria-not good at all. 
If you have used anti-bacterial soaps, discontinue their use and have your septic tank thoroughly purged.