They are two separate ingredients. One feeds the lawn,( a fertilizer) the other is a broad leaf herbicide. All in one product. Scotts is an excellent product line.

Selective herbicides that kill one plant species and not others work by destroying molecular compounds in those plant species. Grasses have a different molecular compound makeup than broad leaf plants. So the herbicide for weed prevention in grass targets broad leaf plant compounds but not grass. The 'feed' in weed and feed is simply fertilizer. The reason you don't want to water too soon after application of chemicals is that the water washes off the chemical before the target plants cell structure can absorb the chemicals. A dry weed and feed chemical prevents weed seed from germinating but doesn't kill existing weeds and needs to be watered to activate the chemical. Liquid spray weed and feed kill existing weeds but normally don't prevent germination of weeds. So you need to apply both types early in the season. The spray contains 24D that is prone to drift. It is very easy to spray your lawn with 24D and have the chemical drift to your or your neighbors trees, shrubs, flowers, or vegetables killing them instead of the weeds in your lawn.