Several things can make a toilet or drain overflow, from a blockage in a toilet itself or in a sewer pipe in the house or on its way to the street if you are not on municipal sewer. If you are on your own septic system, there are many other things that can go wrong. To find the problem with your sewer drains and/or septic system go to the Septic System Troubleshooting Guide and follow it down until it leads you to your problem.
If you are totally unfamiliar with septic systems, go to the Evolution of the Septic System to find out how they came to be and gain an understanding of the different system components.