Toilet occasionally runs shortly after flushing
Usually you can adjust the float so that it shuts off at a lower water level. Also, sometimes the float is just getting stuck (and it needs to be cleaned). 



Edit: If adjustment or cleaning does not work, then you need to replace the fill valve (it's not an expensive part).

Edit2: Reiterating Ed Beal's comment- you may want to start with turning off the water to the toilet and flushing the water out. Then you can inspect or replace the fill valve. A fill valve kit can currently be had for under $10.