Why is water running from the cystern into the tiolet bowl all the time?

After flushing the tiolet the cystern duly fills up with water. The water from the cystern then leaks into the tiolet bowl causing the cystern to have to fill up time and time again. I have turned off the valve supplying the water to the cystern in the short term but how do I stop this leak from the cystern into the bowl, I don't know how its happening.... Many thanks