There are many reasons why a toilet can leak as well as a few different ways they leak. The most common type of toilet leak is "a running toilet," which is when water leaks from the tank's flush valve into the bowl. The cause is usually associated with the flapper valve, a pliable rubber valve that is connected by chain to the flush handle. Over time, the soft rubber of the flapper valve will become firm and it might even warp. This prevents the flapper from making a good seal against the flush valve seat, allowing water to leak into the bowl. It's usually a simple fix to replace the flapper valve if you buy a matching replacement. Simply turn off the water supply beneath the toilet. Next, remove the old flapper from the connection pins on the flush valve then disconnect it from the flush handle chain. Put the new one on in its place, making the same connections at the flush valve and chain. Turn on the water supply valve and do a test flush or two.