Why is my toilet backing up?
The problem is probably in one of these places:


Between the toilet and the house's sewer pipe. This is the most common place for stoppages, and can usually be fixed with a plunger. If a plunger won't do it, then a flexible cable or spring can be run down to clear it out. A foreign object, such as a child's toy that has been flushed, can cause recurrent stoppages here.
The sewer line between the house and the sewer or septic system. Sometimes tree roots grow into this pipe, which collect toilet paper, etc. can cause a blockage. This can be cleared out with a roto rooter or similar device. Also, this can freeze if the weather conditions are right. For example, if there is less snow than usual (for insulation) and unusually cold temperatures, this can allow it to freeze.
The local sewer system. Sometimes the local sewer can have back pressure, either from a storm sewer leaking into the septic sewer or other problems. This is more likely if the house is in a low area. There should be a valve to prevent the sewage from backing up to the house, but this valve will also prevent sewage from leaving the house when it is closed.