Running toilet after flushing
Generally this is either a leaking flapper valve, or a leaking fill valve.  



Look in the tank.  There is a tube that sticks up above the normal full water level.  This is the "overflow tube" which prevents the tank from overflowing - it connects straight to the bowl.  

When you flush it (DON'T), the tank quickly drains, then it refills up to the normal level, maybe an inch below the top of the overflow tube. 


If the flapper valve is leaking, water will slip below normal until the fill valve kicks in to bring it back. 
If the fill valve is leaking, water will fill to normal, then slowly fill above normal right up to the top of the overflow pipe.  


So, if you find the water at the top of the overflow pipe, it's the fill valve.

Otherwise, we don't know. Maybe you just flushed it.  Find other tasks in the bathroom.  If it's the flapper, you'll occasionally hear the valve do a moment of refilling.  If it's the valve, the water will slowly rise.