It is actually one of the most common household plumbing issues that everyone of us has run into at one time or another.
Most of the times, it is a faulty component like the flapper that causes this problem.
The flapper is a part of the tank that controls the flow of water into the bowl. It may not close properly at times, letting water run down constantly.
On the other hand, there may a host of other problems behind this issue. Here are some of the most common ones: