Weed preventer like Preen or Corn Gluten prevents weed seeds (or any seeds for that matter) from germinating but will not kill existing weeds that already have a root system.  Weed killers DO kill existing weeds but won’t stop seeds from germinating later.  There are weed killers on the lawn that will kill the weeds but not the grass and there are plant killers like Round-up and Kleen-up that kill ANY PLANT that they are sprayed on including your lawn or flowers…use these with caution.